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

Explanation

Let the present ages of the father and son be F and S. Ten years ago, F − 10 = 3(S − 10), so F = 3S − 20. Ten years later, F + 10 = 2(S + 10), so F = 2S + 10. Hence S = 30 and F = 70. Their present-age ratio is 70:30 = 7:3.