Three friends go to a restaurant for lunch. When the bill arrives it comes to exactly £30. So each person puts in £10 towards the bill. The waiter takes the money to the till and the manager points out that the customers have been overcharged by £5. The dishonest waiter takes 5 £1 coins from the till and keeps 2 for himself, giving the other 3 back to the customers.

Each of the friends now has paid £10 minus the £1 they got back for the meal i.e. each paid £9 which in total is £27 for all 3 friends. The waiter has kept £2 making a total of £27 + £2 = £29 - so what happened to the extra £1 coin?.