3 people (pirates, if you read the same book I did!) get a room at an Inn and each pay $10. The Inn Keeper realizes the room was only $25 and that he overcharged them. He tells the clerk to give $5 back to the people. Seeing that the people wouldn't be able split the change evenly, and given that they weren't expecting the money back any way, the clerk pockets $2 and gives the 3 people $3.
Each receives $1 back, and one says: sweet! we got the room for $27 (3 x $9). The clerk still has $2 in his pocket.