Question 8150
Let present age of son            = X years
So, Present age of father        = 4X years 

Age of son after 18 years         = X+18 years
Age of father after 18 years     = 4X+18 years ……… (i)
Father will be twice as old his son after 18 years 
         i.e. 2 (X+18) ……….(ii)

Equating age of father i.e. (i) and (ii)

4X+18 = 2(X+18)
4X+18 = 2X + 36
4X – 2X = 36 – 18
2X           = 18
X            = 9  

So the present age of son and father is 9 years and 4 x 9= 36 years respectively.

Verify- Fathers age will be twice the age of son after 18 years.

Age of son after 18 years  =  9+ 18  = 27 years
Age of son after 18 years  =  36 + 18 = 54 years which is twice of 27 years.