A tricky logical (age) problem

It's an old question, may be from 7th grade, but I am really looking for a good explanation for this question:

A says to B, "I am three times as old as you were, when I was as old as you are". If the sum of their present ages is 60, find the ages of A and B respectively.

via Recent Questions - Mathematics - Stack Exchange http://math.stackexchange.com/questions/294427/a-tricky-logical-age-problem