The Rainbow Hotel

Riddle: 3 guys are checking into a hotel. The guy at the front desk says it's 30 dollars for one room. So they each pay 10 dollars each. When they get to the room, the manerger knocks on their door and says you should've never paid 30 dollars tonight because it's 5 dollar discount week. So they go back downstairs to the front desk to get back their 5 dollars. The guy at the front desk doesn't know how to split 5 dollars between 3 people, so he keeps 2 dollars and gives each of them a dollar back. If this is so, then this means that instead of them paying 10 dollars each, they paid 9 dollars each. 9 dollars plus the 3 guys equals 27 dollars. Plus the 2 dollars that the guy at the front desk kept, equals 29 dollars...Where'd the other dollar go??