It's a trick question. The three guys paid $27 ($9 each), and $2 went to the clerk as a tip and $25 went to the hotel. That's all there is to it. The trick is that they make you think the $2 and the $27 spent should sum up to 30. Actually, the $2 is part of the $27, and it's the $3 they retain (each $1) and the $27 that should sum up to $30 (and, of course, 27+3=30). There's no missing $1.
Exactly what durvasa said. They each spent $9...that's all there is to it. The tip's already in there. There's no "extra" $2 that needs to be added. They spent $30, and each got $1 back...so they spent $27 total. The $27 that went to the hotel = $25 bill + $2 tip.
This makes complete sense. Kindergardeners would be more likely to give up and say they can't answer it while Harvard grads would assume that there's an answer and try to find one.
