SOLUTION: The age of father is twice that of the elder son . Ten years hence the age of father will be be three times that of younger son . If the difference between of ages of the two sons

Let "x" be the present age of the elder son.
Then the father's present age is 2x, while the younger son's present age is (x-15).

In 10 years: The father's age will be      (2x)+10.
             the younger son's age will be (x-15) + 10 = x-5.

From the other side, in ten years the age of the father will be 3(x-5).

Thus you get an equation

2x + 10 = 3(x-5).

Simplify and solve for x:

2x + 10 = 3x - 15  --->  10 + 15 = 3x - 2x  --->  x = 25.