Math riddle

So our Math teacher told us this one and i thought i would see if you guys could get it. It's a three Studges Riddle so hopefully you guys know what that classic comidy is.

So Larry, Curly, and Moe go and stay at a hotel for the night and ask the front desk clerk for a room. HE says it's $30's for one night so all three of them hand over ten dollars then they head up to the room. The desk clerk calls over the bell boy to carry their lugage up to the room and realizes that he made them pay to much so he makes the bell boy give them $5's back. On the way up to the room he figures that he'll give them three dollars and keep the other two so he goes up to the room and knocks on the door. Curly opens the door and the bell boy says the desk clerk made you pay too much here's three dollars back. Curly says thank you and takes the they started out giving the clerk $30 to start with subtract $3 from that and what do you get? Now add the two dollars that the bell boy kept to that and what do you get? Now that question is what happened to the other dollar?