Working backwards from a mean
Working backwards from a mean of five numbers to find the missing fifth value.
The Question
The mean of five numbers is 12. Four of them are 8, 15, 11 and 9. What is the fifth number?
The Answer
Reveal the Answer
A mean of 12 across five numbers means the total is 5 × 12 = 60. The four given add to 8 + 15 + 11 + 9 = 43. So the fifth is 60 − 43 = 17. Go to the TOTAL first — the mean on its own tells you nothing until you multiply it back up.
Now watch the solution
Practice Sheet + Marked Answers
The four-question practice sheet, the solutions in our teaching voice and a self-marking quiz for this question are part of The Question Desk course — every school day of the term, with progress you can see. Enrolled group students get it included.
Want a Plan Built Around Your Child, Not Just This Week’s Question?
Small groups of around 4–5 (max 8), matched to your child’s year group and pace. Start with the free baseline assessment and we’ll tell you exactly where they are.
