<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Constant Difference Principle</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
    }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: transparent;
      color: #1e2945;
      padding: 10px 8px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: 100%;
    }

    .header-badge {
      background: #1e2945;
      color: #ffffff;
      font-size: 13px;
      font-weight: 700;
      padding: 6px 14px;
      border-radius: 20px;
      letter-spacing: 0.5px;
      text-transform: uppercase;
      margin-bottom: 12px;
      display: inline-flex;
      align-items: center;
      gap: 6px;
    }

    .grid-container {
      display: grid;
      grid-template-columns: 1fr auto 1fr;
      gap: 12px;
      width: 100%;
      max-width: 580px;
      align-items: center;
    }

    .method-card {
      background: #ffffff;
      border-radius: 12px;
      padding: 12px 10px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      border: 1.5px solid #e2e8f0;
      box-shadow: 0 2px 6px rgba(0,0,0,0.04);
      min-width: 0;
    }

    .method-card.standard {
      border-color: #fecaca;
      background: #fffafa;
    }

    .method-card.smart {
      border-color: #bbf7d0;
      background: #f0fdf4;
    }

    .card-title {
      font-size: 11px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      margin-bottom: 4px;
      white-space: nowrap;
    }

    .tag-messy {
      color: #dc2626;
      background: #fee2e2;
      padding: 3px 8px;
      border-radius: 6px;
    }

    .tag-smart {
      color: #16a34a;
      background: #dcfce7;
      padding: 3px 8px;
      border-radius: 6px;
    }

    .math-block {
      font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Courier New", monospace;
      font-size: 20px;
      font-weight: 700;
      line-height: 1.35;
      margin: 6px 0;
      user-select: none;
      white-space: nowrap;
      width: fit-content;
    }

    .math-row {
      display: flex;
      justify-content: flex-end;
      align-items: center;
      white-space: nowrap;
      letter-spacing: 4px;
    }

    .math-op {
      margin-right: 6px;
      letter-spacing: normal;
    }

    .borrows {
      font-size: 12px;
      color: #dc2626;
      height: 16px;
      line-height: 16px;
      display: flex;
      justify-content: flex-end;
      padding-right: 2px;
      letter-spacing: 4px;
    }

    .borrows-spacer {
      visibility: hidden;
    }

    .struck {
      position: relative;
      display: inline-block;
      color: #64748b;
    }

    .struck::after {
      content: "";
      position: absolute;
      left: -1px;
      top: 50%;
      width: 14px;
      height: 2px;
      background: #dc2626;
      transform: rotate(-45deg);
    }

    .math-line {
      border-bottom: 2px solid #1e2945;
      margin: 4px 0;
      width: 100%;
    }

    .diff-result {
      color: #1e2945;
      font-weight: 800;
    }

    .result-smart {
      color: #15803d;
    }

    /* Shift Connector */
    .shift-arrow {
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      gap: 6px;
      padding: 0 4px;
    }

    .shift-pill {
      background: #1e2945;
      color: #ffffff;
      font-size: 11px;
      font-weight: 700;
      padding: 4px 10px;
      border-radius: 12px;
      white-space: nowrap;
      letter-spacing: 0.3px;
    }

    .arrow-icon {
      font-size: 20px;
      color: #1e2945;
      font-weight: bold;
      line-height: 1;
      display: inline-block;
      transition: transform 0.2s ease;
    }

    .status-badge {
      font-size: 11px;
      font-weight: 700;
      text-align: center;
      padding: 4px 8px;
      border-radius: 6px;
      width: 100%;
      margin-top: 6px;
      white-space: nowrap;
    }

    .status-badge.messy {
      background: #fee2e2;
      color: #991b1b;
    }

    .status-badge.clean {
      background: #bbf7d0;
      color: #14532d;
    }

    /* Interactive Controls */
    .interactive-bar {
      margin-top: 12px;
      display: flex;
      align-items: center;
      gap: 8px;
    }

    .btn-step {
      background: #1e2945;
      color: #ffffff;
      border: none;
      padding: 7px 16px;
      border-radius: 8px;
      font-size: 12px;
      font-weight: 600;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      gap: 6px;
      transition: background-color 0.2s ease, box-shadow 0.2s ease;
    }

    .btn-step:hover {
      background: #334155;
    }

    .col-highlight {
      display: inline-block;
      border-radius: 4px;
      transition: background-color 0.2s ease, color 0.2s ease;
    }

    .active-col {
      background: #fef08a;
      color: #854d0e;
      padding: 0 1px;
    }

    /* Responsive Mobile Layout */
    @media (max-width: 480px) {
      .grid-container {
        grid-template-columns: 1fr;
        gap: 8px;
        max-width: 330px;
      }

      .shift-arrow {
        flex-direction: row;
        gap: 8px;
        padding: 2px 0;
      }

      .arrow-icon {
        transform: rotate(90deg);
      }

      .math-block {
        font-size: 19px;
      }
    }
  </style>
</head>
<body>

  <div class="header-badge">
    <span>✦</span> Constant Difference Principle
  </div>

  <div class="grid-container">
    
    <div class="method-card standard">
      <div class="card-title tag-messy">Standard Form</div>
      <div class="math-block">
        <div class="borrows">
          <span>7</span>&nbsp;<span>9</span>&nbsp;<span>9</span>&nbsp;<span style="font-size:10px;">10</span>
        </div>
        <div class="math-row">
          <span class="struck">8</span>&nbsp;<span class="struck">0</span>&nbsp;<span class="struck">0</span>&nbsp;<span class="struck">0</span>
        </div>
        <div class="math-row">
          <span class="math-op">−</span><span>3</span>&nbsp;<span>6</span>&nbsp;<span>8</span>&nbsp;<span>7</span>
        </div>
        <div class="math-line"></div>
        <div class="math-row diff-result">
          <span>4</span>&nbsp;<span>3</span>&nbsp;<span>1</span>&nbsp;<span>3</span>
        </div>
      </div>
      <div class="status-badge messy">Complicated Regrouping</div>
    </div>

    
    <div class="shift-arrow">
      <span class="shift-pill">−1 to both</span>
      <span class="arrow-icon">➔</span>
    </div>

    
    <div class="method-card smart">
      <div class="card-title tag-smart">Constant Difference</div>
      <div class="math-block">
        <div class="borrows borrows-spacer">
          <span>·</span>
        </div>
        <div class="math-row">
          <span id="d3" class="col-highlight">7</span>&nbsp;<span id="d2" class="col-highlight">9</span>&nbsp;<span id="d1" class="col-highlight">9</span>&nbsp;<span id="d0" class="col-highlight">9</span>
        </div>
        <div class="math-row">
          <span class="math-op">−</span><span id="s3" class="col-highlight">3</span>&nbsp;<span id="s2" class="col-highlight">6</span>&nbsp;<span id="s1" class="col-highlight">8</span>&nbsp;<span id="s0" class="col-highlight">6</span>
        </div>
        <div class="math-line"></div>
        <div class="math-row diff-result result-smart">
          <span id="r3" class="col-highlight">4</span>&nbsp;<span id="r2" class="col-highlight">3</span>&nbsp;<span id="r1" class="col-highlight">1</span>&nbsp;<span id="r0" class="col-highlight">3</span>
        </div>
      </div>
      <div class="status-badge clean">Zero Regrouping Needed!</div>
    </div>
  </div>

  <div class="interactive-bar">
    <button class="btn-step" id="stepBtn" onclick="cycleStep()">
      <span id="btnIcon">▶</span> <span id="btnText">Trace Place Values</span>
    </button>
  </div>

  <script>
    let currentStep = -1;

    function highlightCol(idx) {
      for (let i = 0; i < 4; i++) {
        const active = (i === idx);
        ['d', 's', 'r'].forEach(p => {
          const el = document.getElementById(p + i);
          if (el) {
            if (active) {
              el.classList.add('active-col');
            } else {
              el.classList.remove('active-col');
            }
          }
        });
      }
    }

    function cycleStep() {
      currentStep = (currentStep + 1) % 5;
      updateUI();
    }

    function updateUI() {
      const btnText = document.getElementById('btnText');
      const btnIcon = document.getElementById('btnIcon');
      const labels = [
        "Ones: 9 − 6 = 3",
        "Tens: 9 − 8 = 1",
        "Hundreds: 9 − 6 = 3",
        "Thousands: 7 − 3 = 4",
        "Trace Place Values"
      ];
      
      if (currentStep >= 0 && currentStep < 4) {
        highlightCol(currentStep);
        btnText.textContent = labels[currentStep];
        btnIcon.textContent = "➔";
      } else {
        highlightCol(-1);
        btnText.textContent = labels[4];
        btnIcon.textContent = "▶";
        currentStep = -1;
      }
    }
  </script>
</body>
</html>

Mental Addition and Subtraction Strategies for Large Numbers

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <title>Decomposition Flow</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    }

    body {
      background: #ffffff;
      color: #1e2945;
      padding: 12px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
    }

    .flow-container {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      gap: 10px;
    }

    .num-row {
      display: flex;
      align-items: center;
      justify-content: space-between;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 8px 12px;
      transition: opacity 0.3s ease, border-color 0.3s ease;
    }

    .main-num {
      font-size: 18px;
      font-weight: 700;
      color: #1e2945;
      min-width: 65px;
    }

    .arrow-split {
      color: #94a3b8;
      font-weight: bold;
      font-size: 15px;
      margin: 0 4px;
    }

    .branches {
      display: flex;
      gap: 6px;
      flex: 1;
      justify-content: flex-end;
    }

    .badge {
      padding: 5px 8px;
      border-radius: 8px;
      font-size: 14px;
      font-weight: 700;
      text-align: center;
      min-width: 58px;
      transition: transform 0.2s ease, box-shadow 0.2s ease;
    }

    /* Place value pastel badges */
    .val-th {
      background: #f3e8ff;
      color: #7e22ce;
      border: 1px solid #d8b4fe;
    }

    .val-h {
      background: #e0f2fe;
      color: #0369a1;
      border: 1px solid #bae6fd;
    }

    .val-t {
      background: #ccfbf1;
      color: #0f766e;
      border: 1px solid #99f6e4;
    }

    .columns-sum {
      display: flex;
      justify-content: flex-end;
      gap: 6px;
      padding-right: 12px;
    }

    .sum-col {
      min-width: 58px;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 4px;
      transition: transform 0.3s ease, opacity 0.3s ease;
    }

    .col-arrow {
      color: #64748b;
      font-size: 13px;
      font-weight: bold;
    }

    .col-result {
      width: 100%;
      padding: 6px 4px;
      border-radius: 8px;
      font-size: 14px;
      font-weight: 800;
      text-align: center;
    }

    .final-bar {
      display: flex;
      align-items: center;
      justify-content: space-between;
      background: #f0fdf4;
      border: 1.5px solid #86efac;
      border-radius: 12px;
      padding: 10px 14px;
      margin-top: 4px;
      transition: all 0.3s ease;
    }

    .final-calc {
      display: flex;
      align-items: center;
      gap: 4px;
      font-size: 14px;
      font-weight: 700;
      color: #1e2945;
      flex-wrap: wrap;
    }

    .final-sum {
      font-size: 19px;
      font-weight: 900;
      color: #15803d;
      background: #dcfce7;
      padding: 3px 10px;
      border-radius: 8px;
    }

    /* Controls & Stepper */
    .controls {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 12px;
      margin-top: 10px;
      width: 100%;
    }

    .step-btn {
      background: #f1f5f9;
      border: 1px solid #cbd5e1;
      color: #334155;
      padding: 6px 14px;
      border-radius: 20px;
      font-size: 13px;
      font-weight: 600;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      gap: 4px;
      user-select: none;
    }

    .step-btn:active {
      background: #e2e8f0;
    }

    .step-dots {
      display: flex;
      gap: 6px;
    }

    .dot {
      width: 8px;
      height: 8px;
      border-radius: 50%;
      background: #cbd5e1;
      cursor: pointer;
      transition: background 0.2s ease, transform 0.2s ease;
    }

    .dot.active {
      background: #2563eb;
      transform: scale(1.25);
    }

    /* Step animations */
    .step-fade {
      opacity: 0.2;
      transition: opacity 0.3s ease;
    }

    .step-fade.active-vis {
      opacity: 1;
    }

    .highlight-active {
      box-shadow: 0 0 0 2px #2563eb;
    }

    @media (max-width: 380px) {
      .main-num {
        font-size: 15px;
        min-width: 50px;
      }
      .badge {
        min-width: 48px;
        font-size: 12px;
        padding: 4px 6px;
      }
      .sum-col {
        min-width: 48px;
      }
      .col-result {
        font-size: 12px;
      }
      .final-calc {
        font-size: 12px;
      }
      .final-sum {
        font-size: 16px;
      }
    }
  </style>
</head>
<body>

  <div class="flow-container">
    
    <div id="row1" class="num-row step-fade active-vis">
      <div class="main-num">4,320</div>
      <div class="arrow-split">→</div>
      <div class="branches">
        <div id="b1-th" class="badge val-th">4,000</div>
        <div id="b1-h" class="badge val-h">300</div>
        <div id="b1-t" class="badge val-t">20</div>
      </div>
    </div>

    
    <div id="row2" class="num-row step-fade active-vis">
      <div class="main-num">+ 2,560</div>
      <div class="arrow-split">→</div>
      <div class="branches">
        <div id="b2-th" class="badge val-th">2,000</div>
        <div id="b2-h" class="badge val-h">500</div>
        <div id="b2-t" class="badge val-t">60</div>
      </div>
    </div>

    
    <div class="columns-sum">
      
      <div id="col-th" class="sum-col step-fade active-vis">
        <div class="col-arrow">↓</div>
        <div class="col-result val-th">6,000</div>
      </div>
      
      <div id="col-h" class="sum-col step-fade active-vis">
        <div class="col-arrow">↓</div>
        <div class="col-result val-h">800</div>
      </div>
      
      <div id="col-t" class="sum-col step-fade active-vis">
        <div class="col-arrow">↓</div>
        <div class="col-result val-t">80</div>
      </div>
    </div>

    
    <div id="row-final" class="final-bar step-fade active-vis">
      <div class="final-calc">
        <span class="badge val-th" style="padding: 2px 6px;">6,000</span> +
        <span class="badge val-h" style="padding: 2px 6px;">800</span> +
        <span class="badge val-t" style="padding: 2px 6px;">80</span>
      </div>
      <div class="final-sum">= 6,880</div>
    </div>
  </div>

  
  <div class="controls">
    <button id="prevBtn" class="step-btn" aria-label="Previous">◀</button>
    <div class="step-dots">
      <div class="dot" data-step="0"></div>
      <div class="dot" data-step="1"></div>
      <div class="dot" data-step="2"></div>
      <div class="dot" data-step="3"></div>
    </div>
    <button id="nextBtn" class="step-btn" aria-label="Next">▶</button>
  </div>

  <script>
    const steps = [
      {
        // Step 0: Show decomposed numbers
        activate: ['row1', 'row2'],
        highlights: []
      },
      {
        // Step 1: Add thousands
        activate: ['row1', 'row2', 'col-th'],
        highlights: ['b1-th', 'b2-th', 'col-th']
      },
      {
        // Step 2: Add all place values
        activate: ['row1', 'row2', 'col-th', 'col-h', 'col-t'],
        highlights: ['b1-h', 'b2-h', 'col-h', 'b1-t', 'b2-t', 'col-t']
      },
      {
        // Step 3: Combine all sums
        activate: ['row1', 'row2', 'col-th', 'col-h', 'col-t', 'row-final'],
        highlights: ['row-final']
      }
    ];

    let currentStep = 3;
    const dots = document.querySelectorAll('.dot');
    const allFadeElements = document.querySelectorAll('.step-fade');
    const allBadges = document.querySelectorAll('.badge, .sum-col, .final-bar');

    function applyStep(idx) {
      currentStep = idx;
      const config = steps[idx];

      // Update dots
      dots.forEach((dot, i) => {
        dot.classList.toggle('active', i === idx);
      });

      // Update visible elements
      allFadeElements.forEach(el => {
        if (config.activate.includes(el.id)) {
          el.classList.add('active-vis');
        } else {
          el.classList.remove('active-vis');
        }
      });

      // Update highlights
      allBadges.forEach(el => el.classList.remove('highlight-active'));
      config.highlights.forEach(id => {
        const el = document.getElementById(id);
        if (el) el.classList.add('highlight-active');
      });
    }

    document.getElementById('prevBtn').addEventListener('click', () => {
      applyStep((currentStep - 1 + steps.length) % steps.length);
    });

    document.getElementById('nextBtn').addEventListener('click', () => {
      applyStep((currentStep + 1) % steps.length);
    });

    dots.forEach((dot, i) => {
      dot.addEventListener('click', () => applyStep(i));
    });

    // Start with all fully visible
    applyStep(3);
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
    }
    body {
      font-family: system-ui, -apple-system, sans-serif;
      color: #1e2945;
      background: transparent;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 12px 16px;
      user-select: none;
    }

    .svg-container {
      width: 100%;
      max-width: 500px;
      flex: 1;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    svg {
      width: 100%;
      height: auto;
      overflow: visible;
    }

    /* Step buttons */
    .controls {
      display: flex;
      gap: 8px;
      margin-top: 10px;
      flex-wrap: wrap;
      justify-content: center;
    }

    .step-btn {
      background: #f1f5f9;
      border: 1px solid #cbd5e1;
      border-radius: 20px;
      padding: 6px 14px;
      font-size: 13px;
      font-weight: 600;
      color: #475569;
      cursor: pointer;
      transition: all 0.25s ease;
    }

    .step-btn:hover {
      background: #e2e8f0;
      color: #1e2945;
    }

    .step-btn.active {
      background: #2563eb;
      border-color: #2563eb;
      color: #ffffff;
      box-shadow: 0 2px 6px rgba(37, 99, 235, 0.3);
    }

    /* SVG transitions */
    .fade-item {
      transition: opacity 0.4s ease, transform 0.4s ease;
      opacity: 0;
      transform-origin: center;
    }

    .fade-item.show {
      opacity: 1;
    }

    .arc-path {
      stroke-dasharray: 400;
      stroke-dashoffset: 400;
      transition: stroke-dashoffset 0.6s ease-in-out, opacity 0.4s ease;
      opacity: 0;
    }

    .arc-path.show {
      stroke-dashoffset: 0;
      opacity: 1;
    }

    .num-pill {
      transition: fill 0.3s ease, stroke 0.3s ease, filter 0.3s ease;
    }
  </style>
</head>
<body>

  <div class="svg-container">
    <svg viewBox="0 0 460 300" aria-label="Mental addition diagram">
      <defs>
        
        <filter id="soft-shadow" x="-8%" y="-8%" width="120%" height="120%">
          <feDropShadow dx="0" dy="2" stdDeviation="3" flood-opacity="0.08" />
        </filter>
        <filter id="glow-shadow" x="-12%" y="-12%" width="130%" height="130%">
          <feDropShadow dx="0" dy="3" stdDeviation="5" flood-color="#2563eb" flood-opacity="0.25" />
        </filter>
      </defs>

      
      
      <path id="arc-blue" class="arc-path" d="M 55,70 C 55,145 275,145 275,70"
            fill="none" stroke="#2563eb" stroke-width="3" stroke-linecap="round" />

      
      <path id="arc-teal" class="arc-path" d="M 165,70 C 165,160 385,160 385,70"
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      <text x="110" y="53" text-anchor="middle" font-size="20" font-weight="600" fill="#94a3b8">+</text>

      
      <g id="box-480">
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        <rect x="115" y="142" width="100" height="38" rx="8" fill="#eff6ff" stroke="#2563eb" stroke-width="2" filter="url(#soft-shadow)"/>
        <text x="165" y="166" text-anchor="middle" font-size="15" font-weight="700" fill="#1d4ed8">= 1,000</text>
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      <g id="badge-teal" class="fade-item">
        <rect x="245" y="142" width="100" height="38" rx="8" fill="#f0fdfa" stroke="#0d9488" stroke-width="2" filter="url(#soft-shadow)"/>
        <text x="295" y="166" text-anchor="middle" font-size="15" font-weight="700" fill="#0f766e">= 1,000</text>
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      <g id="merge-lines" class="fade-item">
        <path d="M 165,185 L 195,220" fill="none" stroke="#94a3b8" stroke-width="2" stroke-dasharray="3,3" />
        <path d="M 295,185 L 265,220" fill="none" stroke="#94a3b8" stroke-width="2" stroke-dasharray="3,3" />
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        <text x="222" y="254" text-anchor="middle" font-size="16" font-weight="700" fill="#5eead4">1,000</text>
        <text x="256" y="254" text-anchor="middle" font-size="16" font-weight="600" fill="#e2e8f0">=</text>
        <text x="300" y="255" text-anchor="middle" font-size="20" font-weight="800" fill="#ffffff">2,000</text>
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  </div>

  
  <div class="controls">
    <button class="step-btn active" onclick="setStep(0)">1. Problem</button>
    <button class="step-btn" onclick="setStep(1)">2. Pair 1,000s</button>
    <button class="step-btn" onclick="setStep(2)">3. Total</button>
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    // Initialize
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    startAutoCycle();
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</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <title>Mental Math Decision Guide</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    }

    body {
      width: 100%;
      background: #ffffff;
      color: #1e2945;
      padding: 12px;
      display: flex;
      flex-direction: column;
      gap: 12px;
      overflow-x: hidden;
    }

    /* 3 Top Decision Cards */
    .cards-row {
      display: grid;
      grid-template-columns: repeat(3, 1fr);
      gap: 8px;
      width: 100%;
    }

    .strategy-card {
      background: #f8fafc;
      border: 2px solid #e2e8f0;
      border-radius: 12px;
      padding: 10px 8px;
      cursor: pointer;
      text-align: center;
      transition: all 0.25s ease;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 4px;
      user-select: none;
    }

    .strategy-card:hover {
      background: #f1f5f9;
    }

    .strategy-card.active {
      border-color: #22b7ff;
      background: #f0f9ff;
      box-shadow: 0 0 0 2px rgba(34, 183, 255, 0.25);
    }

    .icon-badge {
      width: 36px;
      height: 36px;
      border-radius: 50%;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 18px;
      margin-bottom: 2px;
    }

    .strategy-card:nth-child(1) .icon-badge { background: #fee2e2; color: #dc2626; }
    .strategy-card:nth-child(2) .icon-badge { background: #fef3c7; color: #d97706; }
    .strategy-card:nth-child(3) .icon-badge { background: #dcfce7; color: #16a34a; }

    .card-trigger {
      font-size: 12px;
      font-weight: 600;
      color: #64748b;
      line-height: 1.2;
    }

    .card-strategy {
      font-size: 13px;
      font-weight: 800;
      color: #0f172a;
      line-height: 1.2;
    }

    /* Dynamic Visual Stage */
    .stage {
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 14px;
      padding: 16px 12px;
      min-height: 220px;
      display: flex;
      flex-direction: column;
      justify-content: center;
      align-items: center;
      position: relative;
    }

    .view-panel {
      width: 100%;
      display: none;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      gap: 14px;
      opacity: 0;
      transition: opacity 0.3s ease;
    }

    .view-panel.active {
      display: flex;
      opacity: 1;
    }

    /* Math Equations & Blocks */
    .math-row {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
      flex-wrap: wrap;
    }

    .num-pill {
      font-size: 18px;
      font-weight: 700;
      padding: 6px 12px;
      border-radius: 8px;
      background: #ffffff;
      border: 1px solid #cbd5e1;
      box-shadow: 0 1px 3px rgba(0,0,0,0.05);
    }

    .num-pill.target {
      background: #eff6ff;
      border-color: #3b82f6;
      color: #1d4ed8;
    }

    .num-pill.pair {
      background: #ecfdf5;
      border-color: #10b981;
      color: #047857;
    }

    .symbol {
      font-size: 18px;
      font-weight: 700;
      color: #94a3b8;
    }

    .transform-arrow {
      font-size: 20px;
      color: #64748b;
    }

    /* Strategy 1: Dial / Adjustment */
    .dial-visual {
      display: flex;
      align-items: center;
      gap: 12px;
      background: #ffffff;
      padding: 10px 16px;
      border-radius: 10px;
      border: 1px dashed #cbd5e1;
    }

    .adjust-tag {
      font-size: 12px;
      font-weight: 700;
      padding: 2px 6px;
      border-radius: 4px;
    }
    .tag-plus { background: #dcfce7; color: #15803d; }
    .tag-minus { background: #fee2e2; color: #b91c1c; }

    /* Strategy 2: Decomposition Blocks */
    .decomp-grid {
      display: flex;
      gap: 8px;
      flex-wrap: wrap;
      justify-content: center;
    }

    .decomp-block {
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 8px;
      padding: 8px 10px;
      text-align: center;
      display: flex;
      flex-direction: column;
      gap: 2px;
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      font-size: 10px;
      font-weight: 700;
      color: #64748b;
      text-transform: uppercase;
    }

    .decomp-val {
      font-size: 15px;
      font-weight: 700;
      color: #0f172a;
    }

    /* Strategy 3: Connect Pairs */
    .pair-bracket {
      display: flex;
      align-items: center;
      gap: 6px;
      padding: 6px 12px;
      background: #ffffff;
      border: 1.5px solid #10b981;
      border-radius: 8px;
    }

    /* Step indicator dots (interactive) */
    .nav-dots {
      display: flex;
      gap: 8px;
      justify-content: center;
      margin-top: 4px;
    }

    .dot-btn {
      width: 10px;
      height: 10px;
      border-radius: 50%;
      background: #cbd5e1;
      border: none;
      cursor: pointer;
      padding: 0;
      transition: background 0.2s ease;
    }

    .dot-btn.active {
      background: #22b7ff;
      transform: scale(1.2);
    }

    @media (max-width: 480px) {
      .cards-row {
        gap: 4px;
      }
      .strategy-card {
        padding: 8px 4px;
      }
      .card-trigger {
        font-size: 10px;
      }
      .card-strategy {
        font-size: 11px;
      }
      .num-pill {
        font-size: 15px;
        padding: 4px 8px;
      }
      .icon-badge {
        width: 30px;
        height: 30px;
        font-size: 15px;
      }
    }
  </style>
</head>
<body>

  
  <div class="cards-row">
    <div class="strategy-card active" onclick="setStrategy(0)" id="card-0">
      <div class="icon-badge">⚖️</div>
      <div class="card-trigger">Near 100 / 1000?</div>
      <div class="card-strategy">Compensation</div>
    </div>

    <div class="strategy-card" onclick="setStrategy(1)" id="card-1">
      <div class="icon-badge">🧩</div>
      <div class="card-trigger">No Carrying?</div>
      <div class="card-strategy">Decomposition</div>
    </div>

    <div class="strategy-card" onclick="setStrategy(2)" id="card-2">
      <div class="icon-badge">🔗</div>
      <div class="card-trigger">3+ Numbers?</div>
      <div class="card-strategy">Compatible Pairs</div>
    </div>
  </div>

  
  <div class="stage">
    
    
    <div class="view-panel active" id="panel-0">
      <div class="math-row">
        <span class="num-pill target">799</span>
        <span class="symbol">+</span>
        <span class="num-pill">254</span>
      </div>

      <div class="dial-visual">
        <span class="adjust-tag tag-plus">+1</span>
        <span class="transform-arrow">➔</span>
        <span class="adjust-tag tag-minus">-1</span>
      </div>

      <div class="math-row">
        <span class="num-pill target" style="background:#dbeafe; font-weight:800;">800</span>
        <span class="symbol">+</span>
        <span class="num-pill" style="background:#f1f5f9;">253</span>
        <span class="symbol">=</span>
        <span class="num-pill target" style="background:#22c55e; color:#fff; border-color:#16a34a;">1,053</span>
      </div>
    </div>

    
    <div class="view-panel" id="panel-1">
      <div class="math-row">
        <span class="num-pill">3,410</span>
        <span class="symbol">+</span>
        <span class="num-pill">2,150</span>
      </div>

      <div class="decomp-grid">
        <div class="decomp-block">
          <span class="decomp-label">Thousands</span>
          <span class="decomp-val">3k + 2k = 5,000</span>
        </div>
        <div class="decomp-block">
          <span class="decomp-label">Hundreds</span>
          <span class="decomp-val">400 + 100 = 500</span>
        </div>
        <div class="decomp-block">
          <span class="decomp-label">Tens</span>
          <span class="decomp-val">10 + 50 = 60</span>
        </div>
      </div>

      <div class="math-row">
        <span class="symbol">Total:</span>
        <span class="num-pill" style="background:#22c55e; color:#fff; border-color:#16a34a; font-weight:800;">5,560</span>
      </div>
    </div>

    
    <div class="view-panel" id="panel-2">
      <div class="math-row">
        <span class="num-pill pair">250</span>
        <span class="symbol">+</span>
        <span class="num-pill">640</span>
        <span class="symbol">+</span>
        <span class="num-pill pair">750</span>
      </div>

      <div class="math-row">
        <div class="pair-bracket">
          <span style="font-weight:700; color:#047857;">250 + 750</span>
          <span class="transform-arrow">➔</span>
          <span style="font-weight:800; color:#047857;">1,000</span>
        </div>
        <span class="symbol">+</span>
        <span class="num-pill">640</span>
      </div>

      <div class="math-row">
        <span class="symbol">=</span>
        <span class="num-pill pair" style="background:#22c55e; color:#fff; border-color:#16a34a; font-weight:800;">1,640</span>
      </div>
    </div>

  </div>

  
  <div class="nav-dots">
    <button class="dot-btn active" onclick="setStrategy(0)" aria-label="Strategy 1"></button>
    <button class="dot-btn" onclick="setStrategy(1)" aria-label="Strategy 2"></button>
    <button class="dot-btn" onclick="setStrategy(2)" aria-label="Strategy 3"></button>
  </div>

  <script>
    let currentIndex = 0;

    function setStrategy(index) {
      currentIndex = index;
      for (let i = 0; i < 3; i++) {
        const card = document.getElementById('card-' + i);
        const panel = document.getElementById('panel-' + i);
        const dot = document.querySelectorAll('.dot-btn')[i];

        if (i === index) {
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          panel.classList.add('active');
          dot.classList.add('active');
        } else {
          card.classList.remove('active');
          panel.classList.remove('active');
          dot.classList.remove('active');
        }
      }
    }
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Compensation for Addition: 398 + 254</title>
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
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      color: #1e2945;
      padding: 16px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: auto;
      user-select: none;
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    .main-wrapper {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 14px;
    }

    .header-section {
      text-align: center;
    }

    .header-title {
      font-size: 13px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.05em;
      color: #475569;
      margin-bottom: 2px;
    }

    .header-eq {
      font-size: 22px;
      font-weight: 800;
      letter-spacing: -0.01em;
      color: #1e2945;
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
    }
    .header-eq .highlight {
      color: #0284c7;
    }

    .stage-container {
      width: 100%;
      display: flex;
      flex-direction: column;
      gap: 10px;
    }

    .step-card {
      position: relative;
      background: #ffffff;
      border: 2px solid #e2e8f0;
      border-radius: 12px;
      padding: 12px 14px;
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 10px;
      transition: border-color 0.25s ease, opacity 0.25s ease, box-shadow 0.25s ease, background-color 0.25s ease;
      opacity: 0.4;
    }

    .step-card.active {
      opacity: 1;
      border-color: #0284c7;
      box-shadow: 0 4px 14px rgba(2, 132, 199, 0.15);
    }

    .step-card.final.active {
      border-color: #059669;
      box-shadow: 0 4px 16px rgba(5, 150, 105, 0.18);
      background: #f0fdf4;
    }

    .step-num {
      width: 28px;
      height: 28px;
      border-radius: 50%;
      background: #e2e8f0;
      color: #334155;
      font-size: 13px;
      font-weight: 800;
      display: flex;
      align-items: center;
      justify-content: center;
      flex-shrink: 0;
      transition: background-color 0.25s, color 0.25s;
    }
    .step-card.active .step-num {
      background: #0284c7;
      color: #ffffff;
    }
    .step-card.final.active .step-num {
      background: #059669;
      color: #ffffff;
    }

    .step-content {
      flex: 1;
      display: flex;
      align-items: center;
      flex-wrap: wrap;
      gap: 6px;
      font-size: 17px;
      font-weight: 700;
      color: #1e2945;
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    .math-expr {
      font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
      font-size: 18px;
      font-weight: 700;
      color: #1e2945;
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    .badge {
      display: inline-flex;
      align-items: center;
      gap: 4px;
      padding: 4px 8px;
      border-radius: 6px;
      font-size: 11px;
      font-weight: 800;
      letter-spacing: 0.02em;
      text-transform: uppercase;
      white-space: nowrap;
      flex-shrink: 0;
    }

    .badge-blue {
      background: #e0f2fe;
      color: #035388;
      border: 1px solid #7dd3fc;
    }

    .badge-gray {
      background: #f1f5f9;
      color: #334155;
      border: 1px solid #cbd5e1;
    }

    .badge-green {
      background: #dcfce7;
      color: #14532d;
      border: 1px solid #86efac;
    }

    .badge-mod {
      font-family: ui-monospace, monospace;
      font-size: 13px;
      font-weight: 800;
      padding: 2px 6px;
      border-radius: 6px;
    }
    .badge-add {
      background: #dbeafe;
      color: #1d4ed8;
      border: 1px solid #bfdbfe;
    }
    .badge-sub {
      background: #fee2e2;
      color: #b91c1c;
      border: 1px solid #fecaca;
    }

    .check-icon {
      font-size: 14px;
      color: #15803d;
      font-weight: 900;
    }

    .step-hint {
      font-size: 12px;
      font-weight: 600;
      color: #475569;
      width: 100%;
      margin-top: 2px;
    }

    /* Controls */
    .controls {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
      width: 100%;
      margin-top: 2px;
      flex-wrap: wrap;
    }

    .btn-step {
      background: #ffffff;
      border: 1.5px solid #cbd5e1;
      color: #1e2945;
      padding: 6px 12px;
      border-radius: 8px;
      font-size: 12px;
      font-weight: 700;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      gap: 4px;
      transition: background-color 0.2s, color 0.2s, border-color 0.2s;
    }

    .btn-step:hover {
      background: #f1f5f9;
      border-color: #94a3b8;
    }

    .btn-step.active {
      background: #1e2945;
      color: #ffffff;
      border-color: #1e2945;
    }

    .play-btn {
      width: 32px;
      height: 32px;
      border-radius: 8px;
      border: 1.5px solid #cbd5e1;
      background: #ffffff;
      color: #1e2945;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 13px;
      cursor: pointer;
      transition: background-color 0.2s, border-color 0.2s;
    }

    .play-btn:hover {
      background: #f1f5f9;
      border-color: #94a3b8;
    }

    @media (max-width: 400px) {
      body {
        padding: 12px 8px;
      }
      .step-card {
        padding: 10px 10px;
        gap: 8px;
      }
      .math-expr {
        font-size: 16px;
      }
      .step-content {
        font-size: 15px;
      }
      .badge {
        font-size: 10px;
        padding: 3px 6px;
      }
      .btn-step {
        padding: 5px 8px;
        font-size: 11px;
      }
    }
  </style>
</head>
<body>

  <div class="main-wrapper">
    <div class="header-section">
      <div class="header-title">Compensation Strategy</div>
      <div class="header-eq">
        <span class="highlight">398</span> + 254 = ?
      </div>
    </div>

    <div class="stage-container">
      
      <div class="step-card" id="card-0">
        <div class="step-num">1</div>
        <div class="step-content">
          <span class="math-expr">398</span>
          <span class="badge-mod badge-add">+2</span>
          <span>→</span>
          <span class="math-expr" style="color:#0369a1; font-weight:800;">400</span>
          <div class="step-hint">Round 398 up to make an easy number</div>
        </div>
        <div class="badge badge-blue">Friendly Number</div>
      </div>

      
      <div class="step-card" id="card-1">
        <div class="step-num">2</div>
        <div class="step-content">
          <span class="math-expr">400 + 254 = <strong style="color:#1e2945;">654</strong></span>
          <div class="step-hint">Add the friendly number with the second part</div>
        </div>
        <div class="badge badge-gray">Easy Sum</div>
      </div>

      
      <div class="step-card final" id="card-2">
        <div class="step-num">3</div>
        <div class="step-content">
          <span class="math-expr">654</span>
          <span class="badge-mod badge-sub">−2</span>
          <span>=</span>
          <span class="math-expr" style="color:#059669; font-weight:800;">652</span>
          <div class="step-hint">Compensate: subtract the 2 we added earlier</div>
        </div>
        <div class="badge badge-green">
          <span class="check-icon">✓</span> Exact Total
        </div>
      </div>
    </div>

    <div class="controls">
      <button class="play-btn" id="playToggle" aria-label="Play/Pause">⏸</button>
      <button class="btn-step" id="btn-0">1: Round</button>
      <button class="btn-step" id="btn-1">2: Add</button>
      <button class="btn-step" id="btn-2">3: Balance</button>
    </div>
  </div>

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      document.getElementById('card-1'),
      document.getElementById('card-2')
    ];
    const buttons = [
      document.getElementById('btn-0'),
      document.getElementById('btn-1'),
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    const playToggle = document.getElementById('playToggle');

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    let timer = null;

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        if (i === index) {
          btn.classList.add('active');
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          btn.classList.remove('active');
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      currentStep = (currentStep + 1) % 3;
      showStep(currentStep);
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      playToggle.textContent = '⏸';
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      timer = setInterval(nextStep, 3200);
    }

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      isPlaying = false;
      playToggle.textContent = '▶';
      if (timer) {
        clearInterval(timer);
        timer = null;
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      btn.addEventListener('click', () => {
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        startAuto();
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    showStep(0);
    startAuto();
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</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
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      background: transparent;
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      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 12px;
      user-select: none;
    }

    .header-eq {
      font-size: 24px;
      font-weight: 700;
      letter-spacing: -0.5px;
      display: flex;
      align-items: center;
      gap: 8px;
    }
    .badge-sub {
      background: #fee2e2;
      color: #dc2626;
      padding: 2px 8px;
      border-radius: 6px;
    }

    .main-stage {
      width: 100%;
      max-width: 480px;
      position: relative;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 16px;
    }

    /* SVG Number Line */
    .numline-wrap {
      width: 100%;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 16px 12px 10px;
    }
    svg {
      width: 100%;
      height: auto;
      overflow: visible;
      display: block;
    }

    .step-callout {
      width: 100%;
      min-height: 64px;
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 10px;
      padding: 10px 14px;
      display: flex;
      align-items: center;
      justify-content: space-between;
      gap: 12px;
      box-shadow: 0 1px 3px rgba(0,0,0,0.05);
    }
    .callout-math {
      font-size: 20px;
      font-weight: 700;
      font-variant-numeric: tabular-nums;
    }
    .callout-desc {
      font-size: 13px;
      font-weight: 600;
      text-align: right;
      color: #64748b;
    }

    /* Step Navigation */
    .controls {
      display: flex;
      align-items: center;
      gap: 8px;
      margin-top: 4px;
    }
    .step-btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      color: #475569;
      font-size: 13px;
      font-weight: 600;
      padding: 6px 12px;
      border-radius: 8px;
      cursor: pointer;
      display: flex;
      align-items: center;
      gap: 4px;
      transition: all 0.2s ease;
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    .step-btn.active {
      background: #1e2945;
      color: #ffffff;
      border-color: #1e2945;
    }
    .play-btn {
      width: 32px;
      height: 32px;
      padding: 0;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      border-radius: 50%;
    }

    /* Dynamic step visibility */
    .jump-red, .jump-green, .pt-550, .pt-553 {
      transition: opacity 0.3s ease, transform 0.3s ease;
    }
  </style>
</head>
<body>

  <div class="header-eq">
    <span>850</span>
    <span>−</span>
    <span class="badge-sub">297</span>
    <span>=</span>
    <span id="finalAnswer" style="color: #16a34a;">?</span>
  </div>

  <div class="main-stage">
    <div class="numline-wrap">
      <svg viewBox="0 0 400 130">
        
        <line x1="30" y1="100" x2="370" y2="100" stroke="#94a3b8" stroke-width="2.5" stroke-linecap="round" />

        
        <circle cx="350" cy="100" r="5" fill="#1e2945" />
        <text x="350" y="122" text-anchor="middle" font-size="14" font-weight="700" fill="#1e2945">850</text>

        
        <g id="step1Vis" class="jump-red">
          <path d="M 350 90 Q 210 20 70 90" fill="none" stroke="#ef4444" stroke-width="3" stroke-dasharray="4 2" />
          <polygon points="70,90 78,80 82,88" fill="#ef4444" />
          <rect x="180" y="18" width="60" height="22" rx="6" fill="#fee2e2" />
          <text x="210" y="34" text-anchor="middle" font-size="13" font-weight="700" fill="#dc2626">−300</text>

          <circle cx="70" cy="100" r="5" fill="#dc2626" />
          <text x="70" y="122" text-anchor="middle" font-size="14" font-weight="700" fill="#dc2626">550</text>
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          <rect x="83" y="52" width="34" height="18" rx="4" fill="#dcfce7" />
          <text x="100" y="65" text-anchor="middle" font-size="11" font-weight="700" fill="#16a34a">+3</text>

          <circle cx="120" cy="100" r="6" fill="#16a34a" />
          <text x="120" y="122" text-anchor="middle" font-size="14" font-weight="800" fill="#16a34a">553</text>
        </g>
      </svg>
    </div>

    <div class="step-callout">
      <div id="mathText" class="callout-math">850 − 300 = 550</div>
      <div id="descText" class="callout-desc">Round 297 → 300</div>
    </div>
  </div>

  <div class="controls">
    <button class="step-btn play-btn" id="playBtn" title="Auto play">⏸</button>
    <button class="step-btn active" data-step="0">1. Round</button>
    <button class="step-btn" data-step="1">2. Check</button>
    <button class="step-btn" data-step="2">3. Add 3</button>
  </div>

  <script>
    const steps = [
      {
        ans: '?',
        math: '<span style="color:#dc2626">850 − 300 = 550</span>',
        desc: 'Subtracted 300<br><span style="color:#64748b;font-size:11px">(Easy jump)</span>',
        redOpacity: 1,
        greenOpacity: 0
      },
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        ans: '?',
        math: '<span style="color:#dc2626">−300</span> vs <span style="color:#1e2945">−297</span>',
        desc: '<span style="color:#dc2626;font-weight:700">Subtracted 3 too much!</span><br>Must give 3 back',
        redOpacity: 1,
        greenOpacity: 0.35
      },
      {
        ans: '553',
        math: '<span style="color:#16a34a">550 + 3 = 553</span>',
        desc: '<span style="color:#16a34a;font-weight:700">Exact Total</span><br>Compensated +3',
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      descText.innerHTML = s.desc;

      step1Vis.style.opacity = s.redOpacity;
      step2Vis.style.opacity = s.greenOpacity;

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        btn.classList.toggle('active', i === index);
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      currentStep = (currentStep + 1) % steps.length;
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      isPlaying = true;
      playBtn.textContent = '⏸';
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    }

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      isPlaying = false;
      playBtn.textContent = '▶';
      clearInterval(timer);
    }

    playBtn.addEventListener('click', () => {
      if (isPlaying) pauseAuto();
      else {
        nextStep();
        startAuto();
      }
    });

    stepButtons.forEach(btn => {
      btn.addEventListener('click', () => {
        pauseAuto();
        render(parseInt(btn.getAttribute('data-step'), 10));
      });
    });

    render(0);
    startAuto();
  </script>
</body>
</html>

OGM.6.NBT.5

Aligned learning resources

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