Slight deviation but another old classic:

3 men go into a restaraunt for a meal. The bill came to £30 so they each pay £10 each. The manager saw that these people were loyal regulars and asked the waiter to knock £5 off the bill. The waiter was dishonest and decided to give them £1 back each and pocketed the remaining £2 for himself.

Now the men each paid £10 and got £1 back so they each paid £9 and the waiter took £2 but £9 x £3 = £27 and £27 + the £2 the waiter took = £29.

What happened to the missing £1?

Jon