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.
