Originally Posted by BFT3K

Sally is S years old; Billy is B years old. Billy was once S years old. How long ago was that? It was B-S years ago. How old was Sally then? She was S-(B-S) or 2S-B years old then. Billy is now twice that old: B = 2(2S-B) = 4S-2B or 3B = 4S. And we also know that S+B=28. We have two equations and two unknowns, we can probably solve for S and B. Sally is 12 and Billy is 16.