18SX+Mathematic problem

A Mother is 21 years older that her son, and in six years time the child
will be 5 times younger that her.

Question: Where is the father?

Don't run away, this question has a mathematical answer; it isn't as hard
as it seems!
Do not look to the answer; you have to do some calculations.
You have to pay special attention to the question Where is the father?
If you do not want to work, just go to the end and read the answer, but it is a good memory nudge to try.

To find the solution we have to make use of Algebra as follows:

Solution:

The child is now X years old and the mother is Y.
We know that the mother is 21 years older therefore

X+21=Y

We know that in six years time the child will be 5 times younger that the mother.
Then:

5 (X+ 6) = Y + 6

We Replace Y by X + 21 and proceed to simplify:

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

The child is now -3/4 of a year, which is equal to -9 months.

We managed to mathematically demonstrate that the mother is at this moment is actually making love.

Then the answer is?
The father is on top of the mother.