He was 20 when his son was born.
He was 40 when he was twice as old.
Let the Present Age of his Son be : S
Given the Man is Six times as his Son
⇒ The Present of Age Man is : 6S
Twenty Years Later :
⇒ The Age of his Son will be : S + 20
⇒ The Age of the Man is : 6S + 20
Given : After 20 years, The Man will be Twice as old as his Son
⇒ 2(S + 20) = 6S + 20
⇒ 2S + 40 = 6S + 20
⇒ 4S = 20
⇒ S = 5
⇒ The Present Age of his Son is : 5
⇒ The Present Age of the Man is : 6S = 6 × 5 = 30
⇒ The Age of Man when his Son was Born is 30 - 5 = 25 years
