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Tutoring abilities

Imagine you get a fun mobile phone so you can call your grandma every weekend!

To make the phone work, we choose a **phone plan**, which is an agreement on how much money to pay and what you get to use.

**Handset Contracts** (often lasting **24/36** months) bundle a brand-new device together with your monthly service.

Alternatively, **SIM-Only** plans provide service for a device you already own, whereas **Pay As You Go** (PAYG) offers flexible top-ups with no fixed term.

Standard **Handset Contracts** bundle a new physical phone with monthly network service over **24/36** months.

**SIM-Only** contracts provide service allowances when you already own your device, while **Pay As You Go** offers flexible top-ups with no ongoing commitment.

We combine upfront fee ($U$), monthly charge ($M$), and contract duration in months ($t$) to find the **Total Cost**:

An **upfront cost** is the money you pay right away on the first day to get the phone in your hands.

A **monthly charge** is the smaller amount of money you pay every single month to keep your phone working.

<!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>Mobile Phone Total Cost</title>
<style>
  * { box-sizing: border-box; margin: 0; padding: 0; font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif; }
  body {
    background: transparent;
    color: #1e2945;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: space-between;
    min-height: 100%;
    padding: 12px;
    gap: 12px;
    user-select: none;
  }

  .calc-header {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
    font-size: 18px;
    font-weight: 700;
    flex-wrap: wrap;
    background: #f8fafc;
    padding: 10px 16px;
    border-radius: 12px;
    width: 100%;
    max-width: 460px;
    box-shadow: 0 1px 3px rgba(0,0,0,0.05);
  }
  .val-u { color: #f59e0b; }
  .val-m { color: #0284c7; }
  .val-t { color: #8b5cf6; }
  .val-tot { color: #10b981; }

  .visual-container {
    width: 100%;
    max-width: 460px;
    display: flex;
    flex-direction: column;
    gap: 8px;
  }

  .cost-bar-frame {
    width: 100%;
    height: 48px;
    background: #f1f5f9;
    border-radius: 10px;
    display: flex;
    overflow: hidden;
    position: relative;
  }

  .bar-upfront {
    background: #f59e0b;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    color: #fff;
    font-weight: 700;
    font-size: 13px;
    transition: width 0.25s ease;
    flex-shrink: 0;
  }

  .bar-monthly-container {
    display: flex;
    height: 100%;
    flex-grow: 1;
  }

  .month-segment {
    flex: 1;
    height: 100%;
    background: #0284c7;
    border-left: 1px solid rgba(255,255,255,0.3);
    transition: opacity 0.2s ease, transform 0.2s ease;
  }

  .month-segment.inactive {
    background: #e2e8f0;
    border-left: 1px solid #cbd5e1;
  }

  .grid-timeline {
    display: grid;
    grid-template-columns: repeat(12, 1fr);
    gap: 4px;
    width: 100%;
  }

  .grid-cell {
    aspect-ratio: 1;
    background: #e2e8f0;
    border-radius: 6px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 11px;
    font-weight: 600;
    color: #64748b;
    cursor: pointer;
    transition: background 0.2s ease, color 0.2s ease;
  }

  .grid-cell.active {
    background: #8b5cf6;
    color: #ffffff;
  }

  .controls-row {
    width: 100%;
    max-width: 460px;
    display: flex;
    flex-direction: column;
    gap: 8px;
    background: #ffffff;
    padding: 12px;
    border-radius: 12px;
    box-shadow: 0 1px 3px rgba(0,0,0,0.06);
  }

  .slider-row {
    display: flex;
    align-items: center;
    gap: 12px;
  }

  .slider-label {
    font-size: 13px;
    font-weight: 600;
    min-width: 70px;
  }

  input[type=range] {
    flex: 1;
    accent-color: #8b5cf6;
    cursor: pointer;
    height: 6px;
  }

  .presets {
    display: flex;
    gap: 8px;
    justify-content: center;
  }

  .btn-preset {
    flex: 1;
    padding: 8px;
    background: #f1f5f9;
    border: none;
    border-radius: 8px;
    font-size: 12px;
    font-weight: 600;
    color: #334155;
    cursor: pointer;
    transition: background 0.2s ease, color 0.2s ease;
  }

  .btn-preset:active {
    opacity: 0.8;
  }

  .btn-preset.selected {
    background: #8b5cf6;
    color: #ffffff;
  }

  .breakdown-row {
    display: flex;
    justify-content: space-between;
    font-size: 12px;
    font-weight: 600;
    color: #64748b;
    width: 100%;
    max-width: 460px;
    padding: 0 4px;
  }
</style>
</head>
<body>

  <div class="calc-header">
    <span class="val-u">£<span id="txt-u">60</span></span>
    <span>+</span>
    <span>(<span class="val-m">£<span id="txt-m">20</span></span> &times; <span class="val-t"><span id="txt-t">12</span>m</span>)</span>
    <span>=</span>
    <span class="val-tot">£<span id="txt-tot">300</span></span>
  </div>

  <div class="visual-container">
    <div class="breakdown-row">
      <span class="val-u">Upfront (U)</span>
      <span class="val-m">Monthly &times; Time (M &times; t)</span>
    </div>

    <div class="cost-bar-frame">
      <div id="bar-upfront" class="bar-upfront" style="width: 20%;">£60</div>
      <div id="monthly-container" class="bar-monthly-container"></div>
    </div>

    <div class="grid-timeline" id="grid-timeline"></div>
  </div>

  <div class="controls-row">
    <div class="slider-row">
      <span class="slider-label val-t">Months: <span id="lbl-t">12</span></span>
      <input type="range" id="slider-t" min="1" max="24" value="12">
    </div>
    <div class="presets">
      <button class="btn-preset" data-u="0" data-m="30">£0 + £30/m</button>
      <button class="btn-preset selected" data-u="60" data-m="20">£60 + £20/m</button>
      <button class="btn-preset" data-u="150" data-m="15">£150 + £15/m</button>
    </div>
  </div>

<script>
  let upfront = 60;
  let monthly = 20;
  let months = 12;

  const txtU = document.getElementById('txt-u');
  const txtM = document.getElementById('txt-m');
  const txtT = document.getElementById('txt-t');
  const txtTot = document.getElementById('txt-tot');
  const lblT = document.getElementById('lbl-t');
  const sliderT = document.getElementById('slider-t');
  const barUpfront = document.getElementById('bar-upfront');
  const monthlyContainer = document.getElementById('monthly-container');
  const gridTimeline = document.getElementById('grid-timeline');
  const presetBtns = document.querySelectorAll('.btn-preset');

  // Build grid markers (up to 24)
  for (let i = 1; i <= 24; i++) {
    const cell = document.createElement('div');
    cell.className = 'grid-cell' + (i <= months ? ' active' : '');
    cell.textContent = i;
    cell.addEventListener('click', () => {
      sliderT.value = i;
      updateState();
    });
    gridTimeline.appendChild(cell);
  }

  // Build segments inside bar
  for (let i = 1; i <= 24; i++) {
    const seg = document.createElement('div');
    seg.className = 'month-segment' + (i <= months ? '' : ' inactive');
    monthlyContainer.appendChild(seg);
  }

  function updateState() {
    months = parseInt(sliderT.value, 10);
    const total = upfront + (monthly * months);

    txtU.textContent = upfront;
    txtM.textContent = monthly;
    txtT.textContent = months;
    txtTot.textContent = total;
    lblT.textContent = months;

    // Proportional bar width for upfront vs monthly total capacity
    const maxPossible = 150 + (30 * 24);
    const uPercent = (upfront / maxPossible) * 100;
    barUpfront.style.width = Math.max(uPercent, upfront > 0 ? 12 : 0) + '%';
    barUpfront.textContent = upfront > 0 ? '£' + upfront : '';

    // Update monthly segments visibility
    const segments = monthlyContainer.children;
    for (let i = 0; i < 24; i++) {
      if (i < months) {
        segments[i].classList.remove('inactive');
      } else {
        segments[i].classList.add('inactive');
      }
    }

    // Update grid timeline
    const cells = gridTimeline.children;
    for (let i = 0; i < 24; i++) {
      if (i < months) {
        cells[i].classList.add('active');
      } else {
        cells[i].classList.remove('active');
      }
    }
  }

  sliderT.addEventListener('input', updateState);

  presetBtns.forEach(btn => {
    btn.addEventListener('click', () => {
      presetBtns.forEach(b => b.classList.remove('selected'));
      btn.classList.add('selected');
      upfront = parseInt(btn.dataset.u, 10);
      monthly = parseInt(btn.dataset.m, 10);
      updateState();
    });
  });

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

The overall price across the plan lifecycle is the **Total Cost**, calculated using the linear formula: $\text{Total Cost} = U + (M \times t)$, where $U$ is upfront cost, $M$ is monthly charge, and $t$ is contract time in months.

Exceeding your allowance incurs out-of-bundle penalty charges, calculated using:

$$C_{\text{total}} = \text{Base Fee} + \text{Excess Units} \times \text{Unit Rate}$$

Here, you pay your fixed **Base Fee** plus a specific **Unit Rate** multiplied by each of your **Excess Units**.

What do you actually get after you pay those coins?

An **allowance** is the set basket of calling minutes and text messages you are allowed to use each month.

If a plan gives you an allowance of 10 call stars, you can talk until all 10 stars are used up.

Allowances cover call minutes, SMS texts, and mobile data measured in megabytes ($\text{MB}$) or gigabytes ($\text{GB}$).

If you exceed these limits, out-of-bundle penalty charges apply: $C_{\text{total}} = \text{Base Fee} + \text{Excess Units} \times \text{Unit Rate}$.

In this formula, your standard monthly charge is the **Base Fee**, and extra activity (**Excess Units**) is billed at an agreed **Unit Rate**.

<!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>Mobile Plan Cost Breakdown</title>
<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%;
  }

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

  /* Plan tabs */
  .tabs {
    display: flex;
    gap: 8px;
    background: #eef2f6;
    padding: 4px;
    border-radius: 10px;
  }
  .tab-btn {
    flex: 1;
    border: none;
    background: transparent;
    padding: 8px 12px;
    font-size: 14px;
    font-weight: 600;
    color: #6b7280;
    border-radius: 8px;
    cursor: pointer;
    transition: all 0.2s ease;
  }
  .tab-btn.active {
    background: #ffffff;
    color: #1e2945;
    box-shadow: 0 2px 4px rgba(0,0,0,0.06);
  }

  /* Slider control */
  .control-card {
    background: #ffffff;
    padding: 12px 16px;
    border-radius: 12px;
    display: flex;
    flex-direction: column;
    gap: 8px;
  }
  .control-label {
    display: flex;
    justify-content: space-between;
    font-size: 14px;
    font-weight: 600;
  }
  .unit-badge {
    color: #0284c7;
    font-weight: 700;
  }
  input[type="range"] {
    width: 100%;
    accent-color: #0284c7;
    cursor: pointer;
  }

  /* Visual cost stack */
  .visual-card {
    background: #ffffff;
    padding: 16px;
    border-radius: 12px;
    display: flex;
    flex-direction: column;
    gap: 14px;
  }

  /* Bar container */
  .bar-track {
    height: 48px;
    background: #f1f5f9;
    border-radius: 10px;
    display: flex;
    overflow: hidden;
    position: relative;
  }
  .bar-segment {
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 700;
    font-size: 14px;
    color: #ffffff;
    transition: width 0.25s ease;
    white-space: nowrap;
    overflow: hidden;
  }
  .bar-base {
    background: #0284c7;
  }
  .bar-excess {
    background: #f59e0b;
  }

  /* Equation display */
  .calc-row {
    display: flex;
    align-items: center;
    justify-content: space-around;
    padding: 10px 8px;
    background: #f8fafc;
    border-radius: 10px;
    font-size: 15px;
    font-weight: 700;
    text-align: center;
  }
  .calc-item {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 2px;
  }
  .calc-sub {
    font-size: 11px;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.5px;
  }
  .sub-base { color: #0284c7; }
  .sub-rate { color: #f59e0b; }
  .sub-total { color: #10b981; }
  .calc-sym {
    color: #9ca3af;
    font-size: 16px;
  }

  /* Grand Total highlight */
  .total-val {
    font-size: 20px;
    color: #10b981;
  }
</style>
</head>
<body>

<div class="diagram-grid">
  
  <div class="tabs">
    <button class="tab-btn active" id="plan1Btn" onclick="setPlan(10, 3, 'Plan A')">Plan A (£10 + £3/GB)</button>
    <button class="tab-btn" id="plan2Btn" onclick="setPlan(20, 1.5, 'Plan B')">Plan B (£20 + £1.50/GB)</button>
  </div>

  
  <div class="control-card">
    <div class="control-label">
      <span>Excess Data:</span>
      <span class="unit-badge" id="unitsText">4 GB</span>
    </div>
    <input type="range" id="unitSlider" min="0" max="10" step="1" value="4" oninput="updateCalculation()">
  </div>

  
  <div class="visual-card">
    
    <div class="bar-track">
      <div class="bar-segment bar-base" id="baseBar" style="width: 45%;">£10</div>
      <div class="bar-segment bar-excess" id="excessBar" style="width: 55%;">£12</div>
    </div>

    
    <div class="calc-row">
      <div class="calc-item">
        <span class="sub-base calc-sub">Base Fee</span>
        <span id="eqBase">£10.00</span>
      </div>
      <span class="calc-sym">+</span>
      <div class="calc-item">
        <span class="sub-rate calc-sub">Rate × Units</span>
        <span id="eqExcess">£3 × 4</span>
      </div>
      <span class="calc-sym">=</span>
      <div class="calc-item">
        <span class="sub-total calc-sub">Total Bill</span>
        <span class="total-val" id="eqTotal">£22.00</span>
      </div>
    </div>
  </div>
</div>

<script>
  let currentBase = 10;
  let currentRate = 3;

  function setPlan(base, rate, name) {
    currentBase = base;
    currentRate = rate;
    document.getElementById('plan1Btn').classList.toggle('active', base === 10);
    document.getElementById('plan2Btn').classList.toggle('active', base === 20);
    updateCalculation();
  }

  function updateCalculation() {
    const units = parseInt(document.getElementById('unitSlider').value, 10);
    const excessCost = units * currentRate;
    const totalCost = currentBase + excessCost;

    // Update Slider text
    document.getElementById('unitsText').textContent = units + " GB";

    // Visual proportional bar (relative to max possible ~£40 scale)
    const maxScale = 40;
    const basePct = (currentBase / maxScale) * 100;
    const excessPct = (excessCost / maxScale) * 100;

    const baseBar = document.getElementById('baseBar');
    const excessBar = document.getElementById('excessBar');

    baseBar.style.width = basePct + "%";
    baseBar.textContent = "£" + currentBase;

    excessBar.style.width = excessPct + "%";
    excessBar.textContent = excessCost > 0 ? "£" + excessCost.toFixed(excessCost % 1 === 0 ? 0 : 2) : "";

    // Update Equation Text
    document.getElementById('eqBase').textContent = "£" + currentBase.toFixed(2);
    document.getElementById('eqExcess').textContent = "£" + (currentRate % 1 === 0 ? currentRate : currentRate.toFixed(2)) + " × " + units;
    document.getElementById('eqTotal').textContent = "£" + totalCost.toFixed(2);
  }

  // Initial draw
  updateCalculation();
</script>

</body>
</html>

How can we look at two different plans to see which one is better for our piggy bank?

Let's compare **Plan Blue** and **Plan Green** to see how much each costs.

<!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; font-family: system-ui, -apple-system, sans-serif; }
    body { padding: 12px; background: transparent; color: #1e2945; display: flex; flex-direction: column; gap: 12px; }

    /* Timeline / Month Selector */
    .controls {
      display: flex;
      justify-content: center;
      align-items: center;
      gap: 6px;
      background: #ffffff;
      padding: 6px 10px;
      border-radius: 12px;
      box-shadow: 0 1px 3px rgba(0,0,0,0.08);
    }
    .month-btn {
      flex: 1;
      max-width: 70px;
      padding: 6px 0;
      border: 1px solid #e6e6e6;
      border-radius: 8px;
      background: #f8f9fa;
      color: #6b7280;
      font-size: 13px;
      font-weight: 600;
      cursor: pointer;
      text-align: center;
      transition: all 0.2s ease;
    }
    .month-btn.active {
      background: #1e2945;
      color: #ffffff;
      border-color: #1e2945;
    }

    /* Comparison Grid */
    .grid {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 10px;
    }

    .plan-card {
      background: #ffffff;
      border-radius: 12px;
      padding: 12px;
      box-shadow: 0 1px 4px rgba(0,0,0,0.06);
      display: flex;
      flex-direction: column;
      align-items: center;
      position: relative;
      transition: box-shadow 0.3s ease;
    }
    .plan-card.cheaper {
      box-shadow: 0 0 0 2px #10b981, 0 4px 12px rgba(16,185,129,0.15);
    }
    .plan-card.tied {
      box-shadow: 0 0 0 2px #f59e0b, 0 4px 12px rgba(245,158,11,0.15);
    }

    .plan-title {
      font-size: 15px;
      font-weight: 700;
      display: flex;
      align-items: center;
      gap: 6px;
    }
    .plan-blue .plan-title { color: #0284c7; }
    .plan-green .plan-title { color: #059669; }

    .plan-rate {
      font-size: 12px;
      color: #6b7280;
      margin-top: 2px;
      margin-bottom: 8px;
    }

    /* Cost Display */
    .total-cost {
      font-size: 26px;
      font-weight: 800;
      margin-bottom: 8px;
      color: #1e2945;
    }

    /* Visual Cost Bar Container */
    .bar-container {
      width: 100%;
      height: 180px;
      background: #f8f9fa;
      border-radius: 8px;
      display: flex;
      flex-direction: column-reverse;
      padding: 6px;
      gap: 3px;
      overflow: hidden;
      border: 1px solid #eef0f2;
    }

    .cost-block {
      width: 100%;
      border-radius: 4px;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 11px;
      font-weight: 700;
      color: #ffffff;
      transition: height 0.3s ease, opacity 0.3s ease;
    }

    .block-upfront {
      background: #94a3b8;
    }
    .plan-blue .block-month {
      background: #0284c7;
    }
    .plan-green .block-month {
      background: #10b981;
    }

    .status-badge {
      margin-top: 8px;
      font-size: 11px;
      font-weight: 700;
      padding: 3px 8px;
      border-radius: 20px;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      opacity: 0;
      transition: opacity 0.2s ease;
    }
    .status-badge.show {
      opacity: 1;
    }
    .badge-cheaper {
      background: #d1fae5;
      color: #065f46;
    }
    .badge-tied {
      background: #fef3c7;
      color: #92400e;
    }
  </style>
</head>
<body>

  
  <div class="controls">
    <button class="month-btn active" data-month="0">Mo 0</button>
    <button class="month-btn" data-month="1">Mo 1</button>
    <button class="month-btn" data-month="2">Mo 2</button>
    <button class="month-btn" data-month="3">Mo 3</button>
    <button class="month-btn" data-month="4">Mo 4</button>
  </div>

  
  <div class="grid">
    
    <div class="plan-card plan-blue" id="cardBlue">
      <div class="plan-title">Plan Blue</div>
      <div class="plan-rate">$0 + $30/mo</div>
      <div class="total-cost" id="totalBlue">$0</div>
      
      <div class="bar-container" id="barsBlue"></div>
      
      <div class="status-badge" id="badgeBlue">Cheaper</div>
    </div>

    
    <div class="plan-card plan-green" id="cardGreen">
      <div class="plan-title">Plan Green</div>
      <div class="plan-rate">$40 + $10/mo</div>
      <div class="total-cost" id="totalGreen">$40</div>
      
      <div class="bar-container" id="barsGreen"></div>
      
      <div class="status-badge" id="badgeGreen">Cheaper</div>
    </div>
  </div>

  <script>
    const plans = {
      blue: { upfront: 0, monthly: 30 },
      green: { upfront: 40, monthly: 10 }
    };

    const maxScaleCost = 140; // Max possible height scale ($120 at month 4)

    function renderMonth(m) {
      const costBlue = plans.blue.upfront + (plans.blue.monthly * m);
      const costGreen = plans.green.upfront + (plans.green.monthly * m);

      document.getElementById('totalBlue').textContent = `$${costBlue}`;
      document.getElementById('totalGreen').textContent = `$${costGreen}`;

      // Render Blue Bars
      const barsBlue = document.getElementById('barsBlue');
      barsBlue.innerHTML = '';
      if (m === 0) {
        barsBlue.innerHTML = '<div style="height:100%; display:flex; align-items:center; justify-content:center; color:#94a3b8; font-size:12px; font-weight:600;">$0 upfront</div>';
      } else {
        for (let i = 1; i <= m; i++) {
          const b = document.createElement('div');
          b.className = 'cost-block block-month';
          b.style.height = `${(plans.blue.monthly / maxScaleCost) * 100}%`;
          b.textContent = '+$30';
          barsBlue.appendChild(b);
        }
      }

      // Render Green Bars
      const barsGreen = document.getElementById('barsGreen');
      barsGreen.innerHTML = '';
      // Upfront block
      const upfront = document.createElement('div');
      upfront.className = 'cost-block block-upfront';
      upfront.style.height = `${(plans.green.upfront / maxScaleCost) * 100}%`;
      upfront.textContent = '$40';
      barsGreen.appendChild(upfront);

      for (let i = 1; i <= m; i++) {
        const b = document.createElement('div');
        b.className = 'cost-block block-month';
        b.style.height = `${(plans.green.monthly / maxScaleCost) * 100}%`;
        b.textContent = '+$10';
        barsGreen.appendChild(b);
      }

      // Highlight Winner
      const cardBlue = document.getElementById('cardBlue');
      const cardGreen = document.getElementById('cardGreen');
      const badgeBlue = document.getElementById('badgeBlue');
      const badgeGreen = document.getElementById('badgeGreen');

      cardBlue.className = 'plan-card plan-blue';
      cardGreen.className = 'plan-card plan-green';
      badgeBlue.className = 'status-badge';
      badgeGreen.className = 'status-badge';

      if (costBlue < costGreen) {
        cardBlue.classList.add('cheaper');
        badgeBlue.textContent = 'Cheaper';
        badgeBlue.className = 'status-badge badge-cheaper show';
      } else if (costGreen < costBlue) {
        cardGreen.classList.add('cheaper');
        badgeGreen.textContent = 'Cheaper';
        badgeGreen.className = 'status-badge badge-cheaper show';
      } else {
        cardBlue.classList.add('tied');
        cardGreen.classList.add('tied');
        badgeBlue.textContent = 'Same Cost';
        badgeGreen.textContent = 'Same Cost';
        badgeBlue.className = 'status-badge badge-tied show';
        badgeGreen.className = 'status-badge badge-tied show';
      }
    }

    const buttons = document.querySelectorAll('.month-btn');
    buttons.forEach(btn => {
      btn.addEventListener('click', () => {
        buttons.forEach(b => b.classList.remove('active'));
        btn.classList.add('active');
        renderMonth(parseInt(btn.dataset.month, 10));
      });
    });

    // Auto-cycle through months cleanly
    let currentM = 0;
    let autoPlay = true;

    function step() {
      if (!autoPlay) return;
      currentM = (currentM + 1) % 5;
      buttons.forEach(b => {
        b.classList.toggle('active', parseInt(b.dataset.month, 10) === currentM);
      });
      renderMonth(currentM);
    }

    let interval = setInterval(step, 3000);

    buttons.forEach(btn => {
      btn.addEventListener('click', () => {
        autoPlay = false;
        clearInterval(interval);
      });
    });

    renderMonth(0);
  </script>
</body>
</html>

Plan Blue starts at $0 on the first day, but Plan Green saves more money after two months because its **monthly charge** is smaller.

Next time you look at a phone plan, check all three parts: the start cost, the monthly cost, and your allowance!

math

Understanding Mobile Phone Contracts and Plans. Compare upfront costs, monthly charges, and included allowances in mobile phone plans.

Third Level Maths and Numeracy: Managing Money, Best Value and Contracts (CfE MNU 3-09a)

Financial Education: Comparing Consumer Contracts, Tariffs and Budgeting in CfE

Your Money Matters: Understanding Mobile Phone Contracts and Personal Finance

Curriculum for Excellence standard MNU 3-09a addresses Third Level (S1–S3, typical ages 11–14) financial mathematics, focusing on sourcing, comparing, and evaluating contracts to determine best value. When teaching this objective, emphasize algebraic modeling through linear equations ($C = U + Mt$), tabular comparative analysis, and scenario-based role play where students audit realistic consumer usage profiles against marketing claims.

Understanding Mobile Phone Contracts and Plans

Types of Phone Plans

Types of Phone Contracts

Paying for a Phone