阅读理解

　　Mr Black was the manager of a hotel in Atlanta．One weekend all of the hotels were full because of a large business meeting being held in the city．On Sunday night, three men came into Mr Black's hotel and asked for rooms．Mr Black told them that there were no more rooms available．The men didn't know what to do because they had no place to stay in．

　　Mr Black wanted to help them．He remembered that there was an empty room at the far end of the first floor－Room 112．It was a very small room, and had rarely been used as a guest room before．So, be asked the three men if they would mind sharing that small room．They replied that they would be very much satisfied so long as there was a room for them to stay in for the night．

　　Mr Black then told them that the room would cost them ＄ 30 in total．On hearing this, each of the three men gave Mr Black ＄ 10 before they left for their room．

　　However, Mr Black soon began to think that ＄ 30 was really too much for that small room．He called his assistant over and said, “Here is ＄ 5．Give it back to the three men in Room 112．Each of them paid me ＄10．That's too much．”

　　The assistant took the money．While he was on the way to that small room, he thought, “How can three men divide ＄ 5?I'll give each of them only ＄ 1 and keep the ＄ 2 left to myself．The men will be happy to get anything back, and I can also make some money that way．After all, Mr Black will never know anything about it．” So, the assistant returned only＄1 to each of the three men．

　　Each man had first paid＄10．After the assistant returned＄1 to him, each man had actually paid only＄9．There were three men．＄9×3＝＄27．The assistant kept ＄2．＄27＋＄2＝＄29．Where is the missing dollar?