I’ve always enjoyed this one. My students usually fight over the correct answer.
Three men go to rent a room. The room is $30. Each man pays with a ten dollar bill.
An hour later, the inn-keeper realizes he has over charged by $5. He gives the bell hop five one dollar bills and sends him up to give back the money.
The bell hop figures a tip is in order so he puts two of the dollars in his shirt pocket and gives each of the men a dollar each.
So…each man paid nine dollars (they each got one back). Three times nine is 27. The bell hop has two bills in his shirt. That’s 29. Where’s the 30th dollar?
(The answer is rather easy, but rough to explain)