A father is four times as old as his son today. After 20 years, he will be twice as old as at the time of birth of his son. What was the father's age at the time of birth of his son?

Let the present age of the son be S and the father be 4S, meaning the father's age at the son's birth was 3S. After 20 years, the father's age will be 4S + 20, which the question states is twice his age at the time of birth, so 4S + 20 = 2(3S). Solving 4S + 20 = 6S gives 2S = 20 and S = 10. The father's age at the time of his son's birth was 3(10) = 30 years.

• Thanks
• Wrong explanation