now for my question, which i unfortunately put on another (now closed) thread.

3 Lads order 3 pizzas, the delivery arives and the cost is £30.00 So they give the delivery boy £10.00 each - that much is fact.

The delivery boy gets back, hands the £30.00 to the cashier, he is told a mistake was made. The pizzas should of been £25.00, not £30.00.

The cashier gives the delivery boy five £1.00 coins and tells him to take it back to the lads who ordered the pizza.
On the way back, the delivery boy has a thought...these lads did not give him a tip.
He thinks that since there is no way to split £5.00 evenly three ways, he will keep two pounds for himself and give them back three pounds - so far quite straight forward right?
He explains about a mix up in the price, and hands the lads the three pounds, then departs with his two pound tip in his pocket. - Now the fun begins!

Remember £30.00-£25.00=£5.00 Right?
£5.00-£3.00=£2.00 Right?
So what's the problem? Well answer this:

Each of the three lads originally gave £10.00.
They each got back £1.00 in change.
That means they paid £9.00 each, which times three is £27.00.
The delivery boy kept £2.00 for a tip.
£27.00 plus £2.00 equals £ 29.00.

Where the hell is the other pound ???