Problem-Solving and Data AnalysisHardFifth value from a known mean
Problem-Solving and Data Analysis practice question
Five friends recorded how many books each read last year. Four of them read 12, 15, 15, and 18 books. If the mean for all five friends was 16 books, how many books did the fifth friend read?
Answer: 20
A mean of 16 across five people means a total of 80. The four known values sum to 60, so the fifth is 80 − 60 = 20.
Common wrong answers
- If you answered 15
- 15 is the mode of the known values, not the missing one.
- If you answered 16
- 16 is the mean itself; the fifth value must pull the total up to 80.
- If you answered 80
- 80 is the total for all five, not the fifth friend's count.
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