I know this was posted here before I think. Buying a gift 3 people throw in $10 each. The gift is 25 1 back to each of the 3 2 left So they each paid $9 9 x 3 = 27 30-27 = 3 left nevermind found an answer http://en.wikipedia.org/wiki/Missing_dollar_paradox

Summary in 50 words Guests pay $30, get $3 refund. The total not refunded (clerk’s $25 + bellhop’s $2) is $27. Add that $27 not refunded to the $3 refunded, you get the expected $30. There’s no reason to add the bellhop’s $2 (already included in the $27), or to leave the $3 out.

That thing has been around for a LONG time. I was taking algebra back in middle school when someone asked the teacher how that whole thing worked out... Talk about trying to explain to people how it all works on a mathematical sense while not ruining the "riddle" part of it. The teacher gave up.