3 old ladies....
......go into a shop and buy a T.V. it costs £30, so they pay a tenner each. The manager says to the assistant "you charged too much, give them a fiver back". The assistant decides to pocket £2 and gives £3 back to the old ladies.
Therefore the old ladies have paid £9 each which adds up to £27, the assistant pocketed £2, that makes £29, where did the other pound go?