The age of father 10 years ago was thrice the age of his son. Ten years hence, father's age will be twice that of his son. The ratio of their present ages is:

Let the ages of father and son 10 years ago be 3x and x years respectively.

Then, (3x + 10) + 10 = 2[(x + 10) + 10]

 3x + 20 = 2x + 40

 x = 20.

 Required ratio = (3x + 10) : (x + 10) = 70 : 30 = 7 : 3.