Odd Msn convo

Mom has weird ways of teaching stuff to me.
Try doing this for example:

一天有个年轻人来到王老板的店里买了一件礼物 这件礼物成本是18元，标价是21元。 结果是这个年轻人掏出100元要买这件礼物。 王老板当时没有零钱，用那100元向街坊换了100元的零钱，找给年轻人79元。 但是街坊後来发现那100元是假钞，王老板无奈还了街坊100元。 现在问题是：王老板在这次交易中到底损失了多少钱

Translation by ME:
This guy went to a shop to buy a present of sorts. The shopkeeper bought the present at $18 and sold it for $21 to the guy. The guy paid in a $100 note, but the shopkeeper had no money, and therefore the shopkeeper changed the note with a passerby and gave a change of $79 to the guy. But it turned out that the note was fake, and the shopkeeper gave the passerby $100 for compensation. Here's the question: How much did the shopkeeper loose?

Help me answer, cos I want to know but Mom keeps putting crosses next to my answers. :P