brain-teaser

John is three times younger than his father, but two times older than his brother Jim. How old is John if Jim is 40 years younger than his father?

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Answer: John is 16 years old.
Let X be John’s age, F be father’s age, and J be Jim’s age. Then, 3X=F, X/2 =J, and J+40=F. Solving this system of three equations for X, we get X=16.

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