Here's a puzzle I heard some time ago, but I never heard the asker's solution. It took me ages to think up a satisfactory solution, but I'm wondering if there are any others you can think of?
100 Marbles Puzzle:
A boy has two bags with marbles in. He has exactly twice as many marbles in one bag as in the other bag. He pours the marbles out and counts them. There are exactly 100 marbles and all the marbles are whole and unbroken.
How many marbles were in each bag to begin with?