What is the relation?

A father is 4 times his son's age. And in 30 years son's age will be half of his father's age. How old are the father and son now???

Answer: 
Son's age : 15
Father's age : 60


Explanation:
The ratio of the father and son's age is now 4 to 1 and 30 years from now it will be 2 to 1. The period of thirty years equals the difference of the two ratios or 2 to 1. Therefore, 30*2 = 60, the father's age and (1/2) * 30 = 15, the son's age.