Three guys get a Hotel room that cost $30. Each give the lady at the desk a 10 dollar bill.
Later the lady realized she had overcharged the men. She gave the bellboy five 1 dollar bills to give to the men. On his way up to the room he decided he could not divide the money evenly so he put two dollars in his pocket and gave each man a dollar bill.
So each man actually paid 9 dollars for the room.
3 * 9 = 27
Plus the 2 dollars the bellboy kept
27 + 2 = 29
What happened to the other dollar?