The sum of the ages of a father and son is 45 years. Five years ago, the product of their ages was four times the fathers age at that time. The present age of father and son, respectively are:

A. 25 years, 10 years
B. 39 years, 6 years
C. 36 years, 9 years
D. None of these
Answer: C

Explanation:  

Let sons age = x years.
Then fathers age = (45 - x)years.
(x - 5)(45 - x - 5) = 4(45- x - 5) hence (x - 5) = 4 so x = 9
Their ages are 36 years and 9 years.