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Tutoring abilities

Imagine you are building a super-long race track for your toy cars. One track piece is **2 meters and 45 centimeters** long, and the next piece is **85 centimeters** long.

How do you find the total length when the numbers have different units attached to them?

A **meter** (m) is a large unit of length, and a **centimeter** (cm) is a tiny unit. Exactly **100 centimeters** fit inside 1 meter.

<!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: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: #f8f9fa;
      color: #1e2945;
      padding: 12px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: 100%;
    }

    .main-view {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      gap: 12px;
    }

    /* Tab switcher */
    .tabs {
      display: flex;
      gap: 6px;
      background: #e2e8f0;
      padding: 4px;
      border-radius: 10px;
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    .tab {
      flex: 1;
      text-align: center;
      padding: 6px 4px;
      font-size: 13px;
      font-weight: 600;
      border-radius: 7px;
      cursor: pointer;
      background: transparent;
      border: none;
      color: #475569;
      transition: background 0.2s, color 0.2s;
    }
    .tab.active {
      background: #ffffff;
      color: #0284c7;
      box-shadow: 0 1px 3px rgba(0,0,0,0.1);
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    /* Work area */
    .stage {
      background: #ffffff;
      border-radius: 12px;
      padding: 14px;
      box-shadow: 0 2px 8px rgba(0,0,0,0.04);
      display: flex;
      flex-direction: column;
      gap: 12px;
    }

    /* Problem statement */
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      align-items: center;
      padding-bottom: 8px;
      border-bottom: 2px dashed #f1f5f9;
      font-size: 17px;
      font-weight: 700;
    }
    .op-tag {
      font-size: 12px;
      font-weight: 700;
      padding: 2px 8px;
      border-radius: 6px;
      background: #e0f2fe;
      color: #0369a1;
    }

    /* Unit columns */
    .columns {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 10px;
    }
    .col-card {
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 8px;
      padding: 10px 8px;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 6px;
    }
    .col-title {
      font-size: 13px;
      font-weight: 700;
      color: #64748b;
      text-transform: uppercase;
      letter-spacing: 0.5px;
    }
    .unit-m { color: #0284c7; }
    .unit-cm { color: #059669; }

    .calc-row {
      font-size: 15px;
      font-weight: 600;
      display: flex;
      justify-content: center;
      align-items: center;
      gap: 4px;
      width: 100%;
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    .calc-divider {
      width: 100%;
      height: 2px;
      background: #cbd5e1;
      margin: 2px 0;
    }
    .calc-total {
      font-size: 16px;
      font-weight: 800;
    }

    /* Regroup animation banner */
    .regroup-banner {
      background: #fef3c7;
      border: 1px solid #fde68a;
      color: #92400e;
      border-radius: 8px;
      padding: 8px 10px;
      font-size: 13px;
      font-weight: 600;
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 6px;
      min-height: 38px;
      transition: all 0.3s ease;
    }
    .regroup-banner.hidden {
      opacity: 0;
      pointer-events: none;
    }

    /* Final Answer Bar */
    .result-bar {
      background: #f0fdf4;
      border: 2px solid #86efac;
      border-radius: 8px;
      padding: 8px 12px;
      display: flex;
      justify-content: space-between;
      align-items: center;
      font-size: 15px;
      font-weight: 700;
      color: #166534;
    }

    /* Stepper controls */
    .stepper {
      display: flex;
      justify-content: space-between;
      align-items: center;
      gap: 8px;
    }
    .step-btn {
      flex: 1;
      padding: 8px 12px;
      background: #0284c7;
      color: #ffffff;
      border: none;
      border-radius: 8px;
      font-size: 13px;
      font-weight: 600;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
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      transition: background 0.2s;
    }
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    .dots {
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    }
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    }
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      background: #0284c7;
      transform: scale(1.2);
    }
  </style>
</head>
<body>

  <div class="main-view">
    <div class="tabs">
      <button class="tab active" onclick="setProblem(0)">Example 1 (+)</button>
      <button class="tab" onclick="setProblem(1)">Regroup (+)</button>
      <button class="tab" onclick="setProblem(2)">Subtract (−)</button>
    </div>

    <div class="stage">
      <div class="problem-header">
        <span id="problemText">1 m 30 cm + 2 m 45 cm</span>
        <span class="op-tag" id="stepTag">Step 1 / 3</span>
      </div>

      <div class="columns">
        
        <div class="col-card">
          <div class="col-title unit-m">Meters (m)</div>
          <div class="calc-row unit-m" id="mRow1">1 m</div>
          <div class="calc-row unit-m" id="mRow2">+ 2 m</div>
          <div class="calc-divider"></div>
          <div class="calc-row calc-total unit-m" id="mTotal">3 m</div>
        </div>

        
        <div class="col-card">
          <div class="col-title unit-cm">Centimeters (cm)</div>
          <div class="calc-row unit-cm" id="cmRow1">30 cm</div>
          <div class="calc-row unit-cm" id="cmRow2">+ 45 cm</div>
          <div class="calc-divider"></div>
          <div class="calc-row calc-total unit-cm" id="cmTotal">75 cm</div>
        </div>
      </div>

      
      <div class="regroup-banner" id="banner">
        <span>⚡ 100 cm = 1 m</span>
      </div>

      
      <div class="result-bar">
        <span>Answer:</span>
        <span id="finalResult">3 m 75 cm</span>
      </div>
    </div>

    
    <div class="stepper">
      <button class="step-btn" id="prevBtn" onclick="prevStep()">← Back</button>
      <div class="dots">
        <span class="dot active" onclick="goToStep(0)"></span>
        <span class="dot" onclick="goToStep(1)"></span>
        <span class="dot" onclick="goToStep(2)"></span>
      </div>
      <button class="step-btn" id="nextBtn" onclick="nextStep()">Next Step →</button>
    </div>
  </div>

  <script>
    const problems = [
      {
        title: "1 m 30 cm + 2 m 45 cm",
        steps: [
          { tag: "1. Match Units", m1: "1 m", m2: "+ 2 m", cm1: "30 cm", cm2: "+ 45 cm", mt: "?", cmt: "?", note: "Group meters & centimeters", ans: "..." },
          { tag: "2. Add Units", m1: "1 m", m2: "+ 2 m", cm1: "30 cm", cm2: "+ 45 cm", mt: "3 m", cmt: "75 cm", note: "30 cm + 45 cm = 75 cm", ans: "3 m 75 cm" },
          { tag: "3. Complete", m1: "1 m", m2: "+ 2 m", cm1: "30 cm", cm2: "+ 45 cm", mt: "3 m", cmt: "75 cm", note: "✓ 75 cm is under 100 cm", ans: "3 m 75 cm" }
        ]
      },
      {
        title: "1 m 80 cm + 2 m 50 cm",
        steps: [
          { tag: "1. Match Units", m1: "1 m", m2: "+ 2 m", cm1: "80 cm", cm2: "+ 50 cm", mt: "?", cmt: "?", note: "Group meters & centimeters", ans: "..." },
          { tag: "2. Add Units", m1: "1 m", m2: "+ 2 m", cm1: "80 cm", cm2: "+ 50 cm", mt: "3 m", cmt: "130 cm", note: "130 cm = 100 cm + 30 cm", ans: "..." },
          { tag: "3. Regroup 100 cm → 1 m", m1: "1 m + 1 m", m2: "+ 2 m", cm1: "80 cm", cm2: "+ 50 cm", mt: "4 m", cmt: "30 cm", note: "⚡ 100 cm becomes +1 m", ans: "4 m 30 cm" }
        ]
      },
      {
        title: "3 m 20 cm − 1 m 70 cm",
        steps: [
          { tag: "1. Match Units", m1: "3 m", m2: "− 1 m", cm1: "20 cm", cm2: "− 70 cm", mt: "?", cmt: "?", note: "20 cm < 70 cm (need more cm)", ans: "..." },
          { tag: "2. Borrow 1 m → 100 cm", m1: "2 m (borrow 1)", m2: "− 1 m", cm1: "120 cm", cm2: "− 70 cm", mt: "2 m", cmt: "120 cm", note: "⚡ 1 m becomes +100 cm", ans: "..." },
          { tag: "3. Subtract Units", m1: "2 m", m2: "− 1 m", cm1: "120 cm", cm2: "− 70 cm", mt: "1 m", cmt: "50 cm", note: "120 − 70 = 50 cm", ans: "1 m 50 cm" }
        ]
      }
    ];

    let currentProb = 0;
    let currentStep = 0;

    function render() {
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      const step = prob.steps[currentStep];

      document.getElementById('problemText').textContent = prob.title;
      document.getElementById('stepTag').textContent = step.tag;

      document.getElementById('mRow1').textContent = step.m1;
      document.getElementById('mRow2').textContent = step.m2;
      document.getElementById('mTotal').textContent = step.mt;

      document.getElementById('cmRow1').textContent = step.cm1;
      document.getElementById('cmRow2').textContent = step.cm2;
      document.getElementById('cmTotal').textContent = step.cmt;

      document.getElementById('banner').textContent = step.note;
      document.getElementById('finalResult').textContent = step.ans;

      // Update dots
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        render();
      }
    }

    function goToStep(idx) {
      currentStep = idx;
      render();
    }

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

We cannot add meters and centimeters directly. Before we add or subtract, we must change everything into the **same unit**.

How do we turn a mixed measurement like 2 m 45 cm into just centimeters?

First, convert the meters into centimeters. Since 1 m is 100 cm, 2 m is $2 \times 100 = 200\text{ cm}$.

Add the extra 45 cm to get $200 + 45 = 245\text{ cm}$.

<!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>Adding Metric Units</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    }

    body {
      background: #f8f9fa;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 16px;
      user-select: none;
    }

    .diagram-area {
      width: 100%;
      max-width: 540px;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 16px;
    }

    /* Equation Display */
    .math-bar {
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 8px;
      font-size: 20px;
      font-weight: 700;
      background: #ffffff;
      padding: 10px 18px;
      border-radius: 12px;
      box-shadow: 0 2px 8px rgba(0, 0, 0, 0.05);
      min-height: 48px;
    }

    .highlight {
      color: #0284c7;
      background: #e0f2fe;
      padding: 2px 8px;
      border-radius: 6px;
      transition: all 0.3s ease;
    }

    .highlight-amber {
      color: #d97706;
      background: #fef3c7;
      padding: 2px 8px;
      border-radius: 6px;
      transition: all 0.3s ease;
    }

    /* Visual Bars Container */
    .bars-stage {
      width: 100%;
      background: #ffffff;
      padding: 20px 16px;
      border-radius: 12px;
      box-shadow: 0 2px 8px rgba(0, 0, 0, 0.05);
      display: flex;
      flex-direction: column;
      gap: 14px;
    }

    .bar-row {
      display: flex;
      flex-direction: column;
      gap: 4px;
    }

    .bar-label {
      font-size: 13px;
      font-weight: 600;
      color: #64748b;
      display: flex;
      justify-content: space-between;
    }

    .track {
      position: relative;
      width: 100%;
      height: 36px;
      background: #f1f5f9;
      border-radius: 8px;
      overflow: hidden;
      display: flex;
    }

    .segment {
      height: 100%;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 13px;
      font-weight: 700;
      color: #ffffff;
      transition: all 0.4s ease;
      white-space: nowrap;
      border-right: 1.5px solid rgba(255, 255, 255, 0.4);
    }

    .seg-meter {
      background: #0284c7;
    }

    .seg-cm {
      background: #38bdf8;
    }

    .seg-add {
      background: #f59e0b;
    }

    .seg-regrouped {
      background: #059669;
    }

    .seg-remain {
      background: #10b981;
    }

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      justify-content: space-between;
      padding: 0 2px;
      font-size: 11px;
      font-weight: 600;
      color: #94a3b8;
    }

    /* Controls */
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      display: flex;
      align-items: center;
      justify-content: center;
      gap: 10px;
      width: 100%;
      max-width: 540px;
      flex-wrap: wrap;
    }

    .step-btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      color: #475569;
      padding: 8px 14px;
      border-radius: 8px;
      font-size: 14px;
      font-weight: 600;
      cursor: pointer;
      display: flex;
      align-items: center;
      gap: 6px;
      transition: background 0.2s, color 0.2s, box-shadow 0.2s;
    }

    .step-btn:hover {
      background: #f1f5f9;
    }

    .step-btn.active {
      background: #0284c7;
      border-color: #0284c7;
      color: #ffffff;
      box-shadow: 0 2px 6px rgba(2, 132, 199, 0.3);
    }

    .play-btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      width: 38px;
      height: 38px;
      border-radius: 8px;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 14px;
      cursor: pointer;
      color: #1e2945;
      transition: background 0.2s;
    }

    .play-btn:hover {
      background: #f1f5f9;
    }
  </style>
</head>
<body>

  <div class="diagram-area">
    
    <div class="math-bar" id="mathDisplay">
      <span>245 cm</span>
      <span>+</span>
      <span class="highlight-amber">85 cm</span>
      <span>=</span>
      <span class="highlight">330 cm</span>
    </div>

    
    <div class="bars-stage">
      
      <div class="bar-row">
        <div class="bar-label">
          <span id="stateTitle">Step 1: Start with 245 cm</span>
          <span id="stateTotal">245 cm</span>
        </div>
        <div class="track" id="barTrack">
          
        </div>
        <div class="ruler">
          <span>0m</span>
          <span>1m (100cm)</span>
          <span>2m (200cm)</span>
          <span>3m (300cm)</span>
          <span>4m</span>
        </div>
      </div>
    </div>
  </div>

  
  <div class="controls">
    <button class="play-btn" id="playBtn" title="Play / Pause">⏸</button>
    <button class="step-btn active" data-step="0">1. Start (245cm)</button>
    <button class="step-btn" data-step="1">2. Add 85cm</button>
    <button class="step-btn" data-step="2">3. Regroup (3m 30cm)</button>
  </div>

  <script>
    const stepsData = [
      {
        title: "Step 1: Start with 245 cm (2 m 45 cm)",
        total: "245 cm",
        math: `<span>245 cm</span> <span style="color:#94a3b8;">+ 85 cm</span>`,
        segments: [
          { width: "25%", text: "1 m", class: "seg-meter" },
          { width: "25%", text: "1 m", class: "seg-meter" },
          { width: "11.25%", text: "45 cm", class: "seg-cm" }
        ]
      },
      {
        title: "Step 2: Add 85 cm (Total = 330 cm)",
        total: "330 cm",
        math: `<span>245 cm</span> + <span class="highlight-amber">85 cm</span> = <span class="highlight">330 cm</span>`,
        segments: [
          { width: "25%", text: "1 m", class: "seg-meter" },
          { width: "25%", text: "1 m", class: "seg-meter" },
          { width: "11.25%", text: "45 cm", class: "seg-cm" },
          { width: "21.25%", text: "+ 85 cm", class: "seg-add" }
        ]
      },
      {
        title: "Step 3: Regroup 300 cm into 3 m",
        total: "3 m 30 cm",
        math: `<span class="highlight">300 cm</span> + 30 cm = <span class="highlight" style="background:#d1fae5;color:#047857;">3 m 30 cm</span>`,
        segments: [
          { width: "25%", text: "1 m", class: "seg-regrouped" },
          { width: "25%", text: "1 m", class: "seg-regrouped" },
          { width: "25%", text: "1 m", class: "seg-regrouped" },
          { width: "7.5%", text: "30 cm", class: "seg-remain" }
        ]
      }
    ];

    let currentStep = 0;
    let isPlaying = true;
    let timer = null;

    const mathDisplay = document.getElementById("mathDisplay");
    const stateTitle = document.getElementById("stateTitle");
    const stateTotal = document.getElementById("stateTotal");
    const barTrack = document.getElementById("barTrack");
    const playBtn = document.getElementById("playBtn");
    const stepBtns = document.querySelectorAll(".step-btn");

    function renderStep(index) {
      currentStep = index;
      const data = stepsData[index];

      stateTitle.textContent = data.title;
      stateTotal.textContent = data.total;
      mathDisplay.innerHTML = data.math;

      // Update segment widths & labels
      barTrack.innerHTML = "";
      data.segments.forEach(seg => {
        const div = document.createElement("div");
        div.className = `segment ${seg.class}`;
        div.style.width = seg.width;
        div.textContent = seg.text;
        barTrack.appendChild(div);
      });

      // Update button states
      stepBtns.forEach((btn, i) => {
        btn.classList.toggle("active", i === index);
      });
    }

    function nextStep() {
      const next = (currentStep + 1) % stepsData.length;
      renderStep(next);
    }

    function startTimer() {
      if (timer) clearInterval(timer);
      timer = setInterval(nextStep, 3500);
      isPlaying = true;
      playBtn.textContent = "⏸";
    }

    function pauseTimer() {
      if (timer) clearInterval(timer);
      timer = null;
      isPlaying = false;
      playBtn.textContent = "▶";
    }

    playBtn.addEventListener("click", () => {
      if (isPlaying) {
        pauseTimer();
      } else {
        startTimer();
      }
    });

    stepBtns.forEach(btn => {
      btn.addEventListener("click", () => {
        pauseTimer();
        const step = parseInt(btn.getAttribute("data-step"), 10);
        renderStep(step);
      });
    });

    // Initialize
    renderStep(0);
    startTimer();
  </script>
</body>
</html>

Now add: $245\text{ cm} + 85\text{ cm} = 330\text{ cm}$. Since 300 cm makes 3 whole meters, our total is **3 m 30 cm**.

What happens when you need to cut a piece of string or subtract length?

Suppose you have a ribbon that is **3 m 10 cm** long, and you snip off **75 cm**.

Convert 3 m 10 cm into centimeters first: $300 + 10 = 310\text{ cm}$.

<!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">
  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.16.9/katex.min.css">
  <script src="https://cdnjs.cloudflare.com/ajax/libs/KaTeX/0.16.9/katex.min.js"></script>
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
      background: transparent;
      color: #1e2945;
      padding: 12px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      user-select: none;
    }

    .nav-tabs {
      display: flex;
      gap: 6px;
      width: 100%;
      max-width: 480px;
      margin-bottom: 12px;
    }

    .tab-btn {
      flex: 1;
      padding: 8px 4px;
      border: 1px solid #e2e8f0;
      background: #ffffff;
      border-radius: 8px;
      font-size: 13px;
      font-weight: 600;
      color: #64748b;
      cursor: pointer;
      text-align: center;
      transition: all 0.2s ease;
      box-shadow: 0 1px 2px rgba(0,0,0,0.03);
    }

    .tab-btn.active {
      background: #22b7ff;
      color: #ffffff;
      border-color: #22b7ff;
      box-shadow: 0 2px 6px rgba(34, 183, 255, 0.3);
    }

    .stage {
      width: 100%;
      max-width: 480px;
      height: 240px;
      background: #ffffff;
      border-radius: 12px;
      border: 1px solid #e6e6e6;
      box-shadow: 0 4px 12px rgba(0,0,0,0.03);
      padding: 16px;
      display: flex;
      flex-direction: column;
      justify-content: space-between;
      position: relative;
      overflow: hidden;
    }

    .math-display {
      font-size: 20px;
      text-align: center;
      color: #1e2945;
      min-height: 32px;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    /* Ribbon Visualizer */
    .ribbon-track {
      width: 100%;
      height: 52px;
      background: #f1f5f9;
      border-radius: 8px;
      position: relative;
      overflow: hidden;
      display: flex;
      border: 1px solid #cbd5e1;
    }

    .ribbon-part {
      height: 100%;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      font-size: 12px;
      font-weight: 700;
      color: white;
      transition: all 0.4s ease;
      position: relative;
    }

    .meter-block {
      background: #0284c7;
      border-right: 1px solid rgba(255, 255, 255, 0.4);
    }

    .cm-block {
      background: #38bdf8;
    }

    .cut-block {
      background: #f43f5e;
      position: relative;
      background-image: repeating-linear-gradient(
        45deg,
        rgba(255,255,255,0.15),
        rgba(255,255,255,0.15) 6px,
        transparent 6px,
        transparent 12px
      );
    }

    .remainder-block {
      background: #10b981;
    }

    .sub-label {
      font-size: 10px;
      font-weight: 500;
      opacity: 0.9;
    }

    .ruler-ticks {
      display: flex;
      justify-content: space-between;
      padding: 0 4px;
      margin-top: 4px;
      font-size: 11px;
      color: #94a3b8;
      font-family: monospace;
    }

    .details-row {
      display: flex;
      justify-content: space-around;
      align-items: center;
      background: #f8fafc;
      border-radius: 8px;
      padding: 10px;
      border: 1px dashed #cbd5e1;
    }

    .stat-pill {
      text-align: center;
    }

    .stat-val {
      font-weight: 700;
      font-size: 16px;
      color: #0284c7;
    }

    .stat-val.cut {
      color: #e11d48;
    }

    .stat-val.result {
      color: #059669;
    }

    .stat-lbl {
      font-size: 11px;
      color: #64748b;
      margin-top: 2px;
    }

    .controls {
      display: flex;
      align-items: center;
      gap: 12px;
      margin-top: 12px;
    }

    .btn-circle {
      width: 38px;
      height: 38px;
      border-radius: 50%;
      border: 1px solid #e2e8f0;
      background: #ffffff;
      color: #1e2945;
      font-size: 14px;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
      transition: background 0.2s, box-shadow 0.2s;
    }

    .btn-circle:hover {
      background: #f1f5f9;
    }

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

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

    .dot.active {
      background: #22b7ff;
      width: 20px;
      border-radius: 4px;
    }
  </style>
</head>
<body>

  
  <div class="nav-tabs">
    <button class="tab-btn active" onclick="setStep(0)">1. Mixed Units</button>
    <button class="tab-btn" onclick="setStep(1)">2. Convert to cm</button>
    <button class="tab-btn" onclick="setStep(2)">3. Subtract</button>
    <button class="tab-btn" onclick="setStep(3)">4. Mixed Result</button>
  </div>

  
  <div class="stage">
    <div id="mathExp" class="math-display"></div>

    <div class="visual-container">
      <div id="ribbonTrack" class="ribbon-track">
        
      </div>
      <div class="ruler-ticks">
        <span>0 cm</span>
        <span>100 cm</span>
        <span>200 cm</span>
        <span>310 cm</span>
      </div>
    </div>

    <div class="details-row" id="detailsRow">
      
    </div>
  </div>

  
  <div class="controls">
    <button class="btn-circle" id="prevBtn" onclick="prevStep()" title="Previous">◀</button>
    <button class="btn-circle" id="playBtn" onclick="togglePlay()" title="Play/Pause">⏸</button>
    <button class="btn-circle" id="nextBtn" onclick="nextStep()" title="Next">▶</button>
    <div class="dots">
      <div class="dot active" onclick="setStep(0)"></div>
      <div class="dot" onclick="setStep(1)"></div>
      <div class="dot" onclick="setStep(2)"></div>
      <div class="dot" onclick="setStep(3)"></div>
    </div>
  </div>

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        stats: `
          <div class="stat-pill"><div class="stat-val">3 m</div><div class="stat-lbl">Meters</div></div>
          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">+</div>
          <div class="stat-pill"><div class="stat-val">10 cm</div><div class="stat-lbl">Centimeters</div></div>
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          <div class="ribbon-part meter-block" style="width: 32.2%; background:#0284c7;">100 cm</div>
          <div class="ribbon-part meter-block" style="width: 32.2%; background:#0284c7;">100 cm</div>
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          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">+</div>
          <div class="stat-pill"><div class="stat-val">10 cm</div><div class="stat-lbl">Extra cm</div></div>
          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">=</div>
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        `
      },
      {
        katex: "310\\text{ cm} - 75\\text{ cm} = 235\\text{ cm}",
        ribbon: `
          <div class="ribbon-part remainder-block" style="width: 75.8%;">
            235 cm
            <span class="sub-label">Remaining</span>
          </div>
          <div class="ribbon-part cut-block" style="width: 24.2%;">
            -75 cm
            <span class="sub-label">Cut Off</span>
          </div>
        `,
        stats: `
          <div class="stat-pill"><div class="stat-val">310 cm</div><div class="stat-lbl">Start</div></div>
          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">−</div>
          <div class="stat-pill"><div class="stat-val cut">75 cm</div><div class="stat-lbl">Subtract</div></div>
          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">=</div>
          <div class="stat-pill"><div class="stat-val result">235 cm</div><div class="stat-lbl">Left</div></div>
        `
      },
      {
        katex: "235\\text{ cm} = \\mathbf{2\\text{ m } 35\\text{ cm}}",
        ribbon: `
          <div class="ribbon-part meter-block" style="width: 32.2%; background:#10b981;">1 m</div>
          <div class="ribbon-part meter-block" style="width: 32.2%; background:#10b981;">1 m</div>
          <div class="ribbon-part cm-block" style="width: 11.4%; background:#34d399;">35 cm</div>
          <div class="ribbon-part" style="width: 24.2%; background:#f1f5f9; color:#94a3b8; font-size:10px; border-left:1px dashed #cbd5e1;">(Cut 75 cm)</div>
        `,
        stats: `
          <div class="stat-pill"><div class="stat-val result">200 cm</div><div class="stat-lbl">2 meters</div></div>
          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">+</div>
          <div class="stat-pill"><div class="stat-val result">35 cm</div><div class="stat-lbl">35 centimeters</div></div>
          <div class="stat-pill" style="font-size:18px;color:#94a3b8;">→</div>
          <div class="stat-pill"><div class="stat-val result">2 m 35 cm</div><div class="stat-lbl">Final Answer</div></div>
        `
      }
    ];

    let currentStep = 0;
    let autoPlayInterval = null;
    let isPlaying = true;

    function renderStep(idx) {
      currentStep = idx;
      const data = stepsData[idx];

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      if (window.katex) {
        katex.render(data.katex, mathEl, { throwOnError: false });
      } else {
        mathEl.textContent = data.katex;
      }

      // Update Ribbon
      document.getElementById('ribbonTrack').innerHTML = data.ribbon;

      // Update Details Row
      document.getElementById('detailsRow').innerHTML = data.stats;

      // Update Tabs & Dots
      document.querySelectorAll('.tab-btn').forEach((btn, i) => {
        btn.classList.toggle('active', i === idx);
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      startTimer();
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</body>
</html>

Now subtract: $310\text{ cm} - 75\text{ cm} = 235\text{ cm}$. That gives us **2 m 35 cm** of ribbon left.

A common mistake is adding the numbers without looking at the units, like saying $2\text{ m} + 85\text{ cm} = 87\text{ m}$.

Always convert to centimeters first so all your numbers speak the **same language** before calculating!

math

Adding and Subtracting Mixed Metric Units of Length. Convert mixed length units (e.g., 2 m 45 cm + 85 cm) into a common unit before calculating.

Converting Metric Units of Length and Solving Measurement Problems (CfE Second Level / KS2)

Converting Metric Units of Length & Word Problems with Mixed Units

Add and Subtract Mixed Metric Units of Length

Pedagogical Note: While the prompt mentions 'Early Level (P1)', the listed benchmark standard (MNU 2-11b) and specific learning objective (converting mixed units like $2\text{ m } 45\text{ cm} + 85\text{ cm}$) strictly map to Curriculum for Excellence Second Level (Primary 5–7 / ages 9–11) in Scotland and Grade 4/5 globally. To teach this successfully: (1) Reinforce the 'Golden Rule of Measures'—never add or subtract numbers carrying different unit labels; (2) Ground conversion using concrete meter sticks and tape measures before moving to abstract column calculations; (3) Emphasise dual representation (e.g., $245\text{ cm} \leftrightarrow 2\text{ m } 45\text{ cm} \leftrightarrow 2.45\text{ m}$) to connect measurement conversion with place-value knowledge.

Adding and Subtracting Mixed Metric Units of Length

The Golden Rule of Length

Adding Mixed Lengths