3 guys go into a shop to buy a TV, its 30 pound so they each pay 10 pound. As they're walking away the shop owner runs after them and told them they paid too much, the TV is only worth 25 pounds. They said for running up he can have 2 pounds and they had a pound each.

they each paid 9 pound for the TV (3x9 = 27)
they gave the shop owner 2 pound

that makes 29 pound. Where did the extra pound dissappear ?