A mother is 21 years older than her child
In 6 years, she'll be 5 times older than her child.
Where is the father?

Making love with the mother.

A mother is 21 years older than her son.
Let the son be S years old
Then the mother is S+21

6 years from now exactly, the mother will be exactly 5 times as old as the child.

S+21+6 = 5(S+6)
S+27 = 5S+30
-3 = 4S
-3/4 = S

So the son is negative three-fourths of a year old, which means he has not been born yet. Since three fourths of a year before he is born is 9 months before he is born, he is just being conceived.