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Spot the mistake relay and maths taboo for IGCSE and GCSE

An end-of-term lesson that is fun and still useful: teams race to find the slip in worked solutions built on the most common exam mistakes, then play maths taboo and pictionary with the year’s vocabulary.

Level
IGCSE and GCSE (Higher and Extended)
Time
39 minutes, plus a 10-minute extension
Topics
Number: percentages, bounds and compound interest; Algebra: brackets, surds and indices; Shape and probability: similar shapes and combined events
Equipment
A calculator is allowed.

Download student sheet (PDF)Answers (PDF)

Suggested timings

PartTimeWhat
Starter5 minQuick questions on the board
Main: task A12 minRelay, part 1
Main: task B12 minRelay, part 2
Main: task C10 minMaths taboo
Extension10 minFast finishers or homework

Starter (5 minutes)

Spot the slip: each line has one mistake.

  1. 5 − 2(x − 3) = 5 − 2x − 6
  2. √8 + √18 = √26
  3. 3 hours 36 minutes = 3.36 hours

Main activity (34 minutes)

Task A: Relay, part 1 (12 min)

Each worked solution has exactly one mistake. Find it, then give the correct answer.

  1. After a 20% increase a price is £84. Find the original price. Working: 20% of 84 = 16.80, so 84 − 16.80 = £67.20.
  2. £2000 is invested at 5% compound interest for 3 years. Find the total. Working: 5% of 2000 = 100; 3 × 100 = 300; total £2300.
  3. Two similar jugs are 10 cm and 15 cm tall. The small one holds 400 ml. How much does the large one hold? Working: scale factor 1.5, so 400 × 1.5 = 600 ml.

Task B: Relay, part 2 (12 min)

Each worked solution has exactly one mistake. Find it, then give the correct answer.

  1. A spinner lands on red with probability 0.3. It is spun twice. Find P(both red). Working: 0.3 + 0.3 = 0.6.
  2. a = 8.4 and b = 2.6, both to 1 decimal place. Find the upper bound of a − b. Working: 8.45 − 2.65 = 5.8.
  3. Solve 4x = 8x − 1. Working: 4x = 8(x − 1), so 4x = 8x − 8 and x = 2.

Task C: Maths taboo (10 min)

Describe the word to your team without saying it or any of the banned words.

  1. Describe PERIMETER without saying: around, edge, outside, length.
  2. Describe PRIME without saying: factor, divide, one, itself.
  3. Describe GRADIENT without saying: slope, steep, line, rise.
  4. Describe MEDIAN without saying: middle, average, centre, order.

Extension (10 minutes)

For fast finishers.

  1. Write −x2 + 6x + 1 in the form a − (x − b)2. Working: −(x2 + 6x) + 1 = −(x + 3)2 + 9 + 1.

For teachers

Teacher notes and full worked answers
  • Relay: teams of three. Give each team the first worked solution; when they bring you the mistake and the corrected answer, they get the next one. The first team through all six wins.
  • Every slip here is one of the common mistakes on our IGCSE Examiner Insights page, in a new example of our own. Show the page afterwards so students can see which mistakes cost the most marks.
  • Taboo cards: one student describes the word without saying it or any of the banned words; their team guesses. The sample clue on each card shows that it can be done.
  • Pictionary (no words or numbers allowed on the board, 60 seconds each): parallel, perpendicular, tangent, histogram, reflection, cuboid, bearing, sector.

Starter

  1. 11 − 2x
    • −2 × −3 = +6, so 5 − 2x + 6 = 11 − 2x
  2. 5√2
    • √8 = 2√2 and √18 = 3√2: simplify first, then add.
  3. 3.6 hours
    • 36 minutes = 36/60 = 0.6 hours

Task A: Relay, part 1

  1. £70
    • Mistake: 20% of the new price was taken. The 20% is of the original.
    • Original × 1.2 = 84, so the original = 84 ÷ 1.2 = £70.
  2. £2315.25
    • Mistake: this is simple interest. Compound interest grows each year.
    • 2000 × 1.053 = £2315.25
  3. 1350 ml
    • Mistake: the length scale factor was used for volume.
    • Volume scale factor 1.53 = 3.375, so 400 × 3.375 = 1350 ml.

Task B: Relay, part 2

  1. 0.09
    • Mistake: adding. For ‘and’ with independent events, multiply.
    • 0.3 × 0.3 = 0.09
  2. 5.9
    • Mistake: for the biggest difference, subtract the smallest b.
    • 8.45 − 2.55 = 5.9
  3. x = 3
    • Mistake: the bases are different, so the powers cannot be compared directly.
    • 22x = 23x − 3, so 2x = 3x − 3 and x = 3.

Task C: Maths taboo

  1. Sample clue: ‘Add up all the sides of a shape.’
    • The word is perimeter. Any clue that avoids the banned words is fine.
  2. Sample clue: ‘Numbers like 2, 3, 5, 7 and 11.’
    • The word is prime. Any clue that avoids the banned words is fine.
  3. Sample clue: ‘How much y goes up when x goes up by 1 on a straight graph.’
    • The word is gradient. Any clue that avoids the banned words is fine.
  4. Sample clue: ‘Put the data in a row from smallest to largest and pick the value halfway along.’
    • The word is median. Any clue that avoids the banned words is fine.

Extension

  1. 10 − (x − 3)2
    • Mistake: −x2 + 6x = −(x2 − 6x), not −(x2 + 6x).
    • −[(x − 3)2 − 9] + 1 = 10 − (x − 3)2, so a = 10 and b = 3.

The same answers are in the answers PDF. Every answer was recomputed by computer before publishing.

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