Three friends check into a motel for the night and the clerk tells them the bill is £30, payable in advance.  So, they each pay the clerk £10 and go to their room.  A few minutes later, the clerk realizes he has made an error and overcharged the trio by £5.  He asks the bellhop to return £5 to the 3 friends who had just checked in.  The bellhop sees this as an opportunity to make £2 as he reasons that the three friends would have a tough time dividing £5 evenly among them; so he decides to tell them that the clerk made a mistake of only £3, giving a pound back to each of the friends.  He pockets the leftover £2 and goes home for the day!  Now, each of the three friends gets a pound back, thus they each paid £9 for the room which is a total of £27 for the night.  We know the bellhop pocketed £2 and adding that to the £27, you get £29, not £30 which was originally spent.  Where did the other pound go???