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A mother is 21 years older than the child. Six years from now the mother will be five times as old as the child. Question: Where’s the father?

Solution:

The mother is 21 years older than the child.
M = C + 21

In six years from now, the mother will be five times as old as the child.
M + 6 = (C + 6) x 5

Hence,
C + 21 + 6 = (C + 6) x 5
C + 27 = 5C + 30
-3 = 4C
C = -3/4

The child is -3/4 years old, that is, it’ll be born in nine months. Thus, by mathematical induction, the father is on top of the mother right now.

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