Maths: Where is the father?

At present the mother is 21 years older than her son.
After 6 years, the age of the mother will be 5 times the age of the son.

Question: Where is the father now?

Solution:
Today:
the son is X years old
and the mother is Y years old:
--- X+21=Y

In 6 years:
--- 5(X+6)=Y+6
-------------------------------------------------
5X+30 = X+21+6
4X = -3
X = -3/4

The son is -3/4 years old. That means -9 months.

So, the father is on top of her ....