Change money first. You have a hundred. Really, you made another fake one. At this point you have * * * 200 yuan. (including a fake 100 yuan). Use this fake one again and exchange it with the neighbor, but the neighbor found out and gave you a real one. (I want to return the leave 100 yuan) After the replacement. You still have 200 yuan, except that 100 yuan is change, and the other one is still fake 100 yuan. In fact, all you can really spend is 100 yuan. Someone bought a gift of 2 1 yuan for you 100 yuan, which is the fake mentioned above. You look for him, 79 yuan. Equivalent to, you lost 79 yuan and donated 2 1 yuan, and the whole lost 100 yuan, but the donated 2 1 yuan, the principal is 18 yuan, and you also earned 3 yuan, so you actually lost 97 yuan.

In fact, when someone gives you fake money, it is equivalent to someone asking you for money, but it is just a nice speech. There is a difference between the cost and the selling price, which should be taken into account.