Three men walk into a hotel looking for rooms. On arrival, the clerk tells them that there is only one room left, but that they can share it for £30. They accept and pay the clerk £30 - £10 each. When the proprietor of the hotel hears that the room has been taken, he realises that the clerk has made a mistake. The room should only have cost the men £25, so the proprietor gives a butler £5 to take to the men. On the way to the room, the butler realises that £5 would be awkward to split between three men, so decides to keep £2 for himself and just give the men £1 each.
The men initially paid £10 each, but have now been given £1 back, meaning they have only paid £9 each. As we know, 3 x £9 is £27, and adding on the £2 that the butler took makes £29.
So, what happened to the extra £1?