Question 13327
let bob's father current  age is x;and that of bob is y
age at four years ago i.e of father=x-4 & of bob =y-4
given 3y=x                                 I
    (x-4)=4*(y-4)                          II
putting value of equ I in II
we have 3y-4=4y-16 i.e y=12 & x=3x12=36