Well after some significant thought, I have decided that I was either not given enough information to solve the puzzle in my last post, or I'm going to need a hint from the person who gave me the puzzle.
Either way though, I don't have an answer to post as I promised I would, but not to worry... I have something better: another puzzle!
This one I heard on NPR's Puzzle Master, so I know it is legit:
It's not a trick question, and there is a simple and elegant solution to the puzzle.
In case you're wondering, I heard this in the car listening to the radio, and I'd estimate that I drove about 5 miles or so before the answers came to me, so I'll guess that that was about 5 minutes. How long did it take you to solve?