Years 2–6 · Proof, critique and metacognition

Reasoning Detective

Find the mathematical error, diagnose it and repair the reasoning.

Play Reasoning Detective

Round 1 of 3

Year

Mode

Challenge code Share the same four digits to get the same challenge.

Calm modeQuieter promptsNothing is timed either way.

Loading the first idea…

Take your time.

If 43 − 18 is written as 35, check the ones: 3 cannot subtract 8 without exchanging one ten.

Create a believable wrong answer and explain how to spot the error.

Year rangeYears 2–6 (KS1–KS2)

Learning objectiveIdentify, explain and correct common errors in primary arithmetic and reasoning.

DifficultyErrors progress from single-step facts to place value, fractions and multi-step reasoning.

Teaching notes and next steps

Useful prerequisite: Explain a calculation using words, symbols or a representation.
Representation: A worked claim is paired with three possible diagnoses and a corrected model.
Common misconception: Treating the final answer as the only evidence and overlooking the method.
Transfer prompt: Create a believable wrong answer and explain how to spot the error.
Adult next step: Use the neutral phrase ‘What could this mathematician try next?’ rather than praising speed.

Reviewed 2026-08-14 by TheLearnDen Editorial Team. The activity does not ask for a name, email address, school, location or account.