Three people check into a hotel.

They pay £30 to the manager and go to their room.

The manager suddenly remembers that the room rate is £25 and gives £5 to the porter to return to the people.

On the way to the room the porter reasons that £5 would be difficult to share among three people so they keep £2 and give £1 to each person.

Now each person paid £10 and got back £1.

So they paid £9 each, totalling £27. The porter has £2, totalling £29.

Where is the missing £1?