A mother is 21 years older than the child. 6 years from now the mother will be 5 times as old as the child.
Question : Where's the father?

Try first, before you check the answer below.
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Solution:

The mother is 21 years older than the child.
M = C + 21
In 6 years from now the mum will be 5 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 9 months.
Thus.. by mathematical induction....
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Right now, the father is on top of the mother...