As I reached the ticket office I realized I had no cash, only a $10 postal order. The fare to Cambridge was $7 and the booking clerk would not accept my postal order. Luckily there was a pawnbroker nearby, so I went in and pawned my postal order for $7. On my way back into the station, I met Patrick, a friend of mine, who offered to buy the pawn ticket from me for $7, to save me the trouble of coming back to London. Naturally I agreed, then rushed in and bought my ticket. When I got to Cambridge I still had $7 of course.
So who lost?
Clue: If the man who travelled to Cambridge had been able to use his $10, he would have had $3 left after deducting the fare. Instead he has $7. So $4 has come from somewhere. In order to redeem the postal order, someone will have to pay the pawnbroker back his $7..
Answer: Patrick is the loser.