The sum of the present ages of a father and his son is 72 years.  When the father was as old as he is now, his age was three times the age of the son at that time.  The ratio of the present ages of the son and the father is?

3 : 5

                Father   :      Son

Present:      x        :        y

 Before*       y        :      2y - x 

*(when mother was as old as her daughter is now) 

Now, ATQ,

⇒ y = 3 ( 2y - x )

⇒ 5y = 3x

⇒ x :  y = 5 : 3

Therefore, ratio of age of son and father = 3 : 5