Four men decide to buy a box of cigars that costs £30.
One man has no money so the other men all put in £10 each.
The man that had no money goes to the shop and buys the cigars, but they have been reduced to £25 in the sale. He pays for them and puts £5 in his pocket.
When he gets back to his friends he gives them all a £1 each change and keeps the other £2 for himself, (total £5 ). So in effect the 3 friends have all paid £9 each, (£10 - £1 paid back) which equals £27 and the other £2 he keeps for himself. Added up this makes £29 . Where is the other pound???