The original puzzle says:
What is the father doing now?
The mother is 21 years older than the son, and in 6 years time she will be 5 times as old as the son. What is the father doing now?
http://goo.gl/ufSkDC
After an easy calculation, you will find that the son's age is -3/4 , which means that he will be born in 9 months. So the parents are supposed to do something in this regard to make it happen.
I tried many phrases (over 30), but still unsuccessful. In the link above you can confirm your answer and check if it is correct.
please let me know if you have any suggestion.