The Missing Dollar

Question: 3 Friends want to rent a motel room. Between the 3 of them, they can only afford 1 room. The room is 30 dollars. They each hand 10 dollars to the clerk. The clerk says the room is only 25 dollars because they are having a sale. So, the clerk hands them 5 one dollar bills. They don't know how to split it evenly without change, so they each keep a dollar and they give the clerk two dollars. Now, if they each spent 10 dollars and got 1 dollar back, then they each spent 9 dollars. 9 x 3 = 27 (nine dollars each) they tipped the clerk 2 dollars. 27 + 2 = 29. Where is the missing dollar.