A father is 9 times as old as his son. In three years, the father will be 6 times as old as his son. How old is the son?

Let the son's age ==S

Father's age ==9S

9S + 3 ==6 [S + 3], solve for S

9S + 3 == 6S + 18

9S - 6S ==18 - 3

3S == 15

S ==15 / 3 == 5 years - the son's age.