Three men go to a hotel and decide to share a room. The room costs $30 per night, so they each leave $10 at reception and head upstairs. Later the concierge recalls that tonight is the twenty-five-dollar special. He gives the bell hop exactly five one-dollar coins(/notes, depends on your currency!). The bell hop arrives at the room and explains the special. The men realise there is no easy way to share the 5 coins between them, so they keep one dollar each and give the bell hop a $2 tip. Later, one of the men exclaims: "Wait, we got $1 back each, right? Well, that means we paid $9 each for the room, right? (Our original $10 each minus the dollar we got back.) Now, three nines is 27, plus the $2 tip gives $29. Where'd the other dollar go!?!" Where, indeed, did the extra dollar go?