Competition problem — Algebra — Maths Olympiad Prep

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Algebra Difficulty 1.6 Junior Find the answer

In the list 7, 9, 10, 11, 18, which number is the average (mean) of the other four numbers?

Your answer

A number or a short expression. Spacing and $ signs are ignored.

Solution

The average of the numbers $7, 9, 10, 11$ is $\frac{7+9+10+11}{4} = \frac{37}{4} = 9.25$ , which is not equal to 18, which is the fifth number. The average of the numbers $7, 9, 10, 18$ is $\frac{7+9+10+18}{4} = \frac{44}{4} = 11$ , which is equal to 11, the remaining fifth number.

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