Let son age is X before 20 years. Which makes father's age as 9X.

20 years back the sum of their ages will be 40 less than 70.

I. E. 70 - 40 = 30.

X + 9X = 30

10X = 30

X= 3

Son.was 3 years before 20 years and father was 27.

Therefore at birth of the son, the father would be 27-3= 24.