After a good meal, the last thing you want to do is think hard, but beware, there could be foul play afoot.
Three friends go to dinner. At the end of the dinner the bill arrives and it is £30 (sterling) :) Each friend puts in a tenner and the waiter goes back to the till... looking at the bill the waiter realises he has made a mistake and it was only £25, as the till is low on change and there is only 3 of them, the waiter gives each man a pound back, and pockets the other £2.
Each man has paid £9, thats £27, plus the £2 the waiter took, thats £29...
What happened to the other £1