Three men go to a hotel and ask how much for a room, the hotelier tells them, �30. They each put ten pounds on the counter and go to the room. The hotelier then realises that he's made a mistake, and the room is in fact �25, so he gives the bell boy 5 �1 coins to put it right. Now the bell boy realises that he can't give each man back the exact right amount, so he decides to give each man back �1, and keep the the other two for himself. This he does. So now we have three men paying �9 each, so 3x9=27, the bell boy kept �2, 3x9=27+2=29 - where has the other �1 gone?