how about this one?

A man and his two flatmates walk into a shop to buy a TV.
They see a TV for sale for #30 and agree that they will club together and pay #10 each.
They approach the salesman who asks his assistant to get the TV from the stock room.
The assistant brings over the TV but realises that the actual price for the TV is #25.
The shop assistant sneakily pockets two pound of this and refunds each flatmate one pound each.
This means that each flatmate has now paid nine pound each for the TV, equalling #27, the shop assistant has pocketed #2,\
So where has the other pound gone???????

They have not paid £9 each they have paid £9.33333 rec.

well he keeps £2 and gives back £3 + the original £25 so the other pound hasn't gone anywhere really