The mum is 21 years older than the child, so...

M = C + 21

In 6 years from now the mum will be 5 times as old as the child.

M + 6 = (C + 6) * 5

(C + 21)+ 6 = (C + 6) * 5

C + 27 = 5C + 30

-3 = 4C

C = -3/4

The child is 3 years 4 months old, i.e. it'll be born in 9 month's time.

Before 4 years and 1 month the father was probably nearby.

Adults will understand it better. :)