A man is at present 9 times the age of his son. In 3 years' time the father's age will be 5 times his son's age. What are the present age of the man and his son?

Solution:

                    y = 9x

                y + 3 = 5(x + 3)

               9x + 3 = 5x + 15

             4x = 12

            x = 3 = son's age

            9x = 27 = father's age