Five numbers have a mean of 12, and four of them are 10, 11, 13 and 14. A pupil answers 36 for the fifth number. Which statement identifies the mistake in this missing-value calculation?
The required total is 60, not 48, so the fifth number is 60 - 48 = 12.✓Bonne réponse
The pupil used the wrong count; the required total is 48, so the fifth number is 48 - 12 = 36.
The pupil was right, because working out 48 - 12 = 36 does recover the number that was missing.
The pupil should have divided rather than subtracted, giving 48 divided by 4.
Explication
Five numbers with a mean of 12 must total 60, and the four known numbers add to 48, so the fifth number is 60 - 48 = 12. Working out 48 - 12 mixes a total and a mean in one subtraction, and a quick check exposes it, because a fifth number as large as 36 would lift the mean of all five well above 12.