A father is three times as old as his son. After fifteen years the father will be twice as old as his son's age at that time. What is the father's present age?

The answer is 45. Suppose the son's age now is S, so the father's age is F=3S. After 15 years, we have F+15=2(S+15). Solving for S we get S=15. Thus, the father's present age is 3S=45 years.