Three merchants come to London in business.  They stay at a hotel and pays 30 pounds for three single rooms.

When the merchants have been helped to their rooms by the piccolo, the portière suddenly realizes the hotel has a special price of 25 pounds for three single rooms this week, so when the piccolo arrives he sends him back up with 5 punds the refund to the merchants.  

In the elevator, on his way up to the merchants' floor, the piccolo realizes he cannot possibly divide five by three, so he finally decides to take two pounds for himself and give one pound each to the merchants.

The merchants have thus each paid 10 minus 1 pound, that is, 9 pounds.