john, twelve years old, is three times as old as his brother. how old will john be when he is twice as old as his brother?

Solution:

        Presently his brother = 12/3 = 4 years

Let after 8 years John will be twice as his brother.

         2(4 + x) = 12 + x

         x = 12 - 4 = 8

Age of john will be = 12 + 8 = 20 years.