First of all, suppose you have 100 yuan in your pocket, and you have 70 yuan left when you buy it (1 20+ 1 50). When the customer came, he gave Master Wang a 50, so now it is (20+50+50). Master Wang changed it and it became (20+50+ 10*5). After the exchange, I found money from 30 yuan and gave it to the customer. Now it is (20+50+ 10*2). After discovering that it was fake money, it was 50 less, and now it is (20+ 10*2). In other words, the original 100 yuan became the present 40 yuan, so * * * lost 60 yuan.

-the method is stupid, but it is absolutely correct.

-Master Wang originally planned to sell at a loss 10 yuan, but now 60 yuan is losing money, and 50 yuan is losing more money than originally thought, so it shouldn't be wrong to answer 50 yuan directly.

In fact, to put it simply, if that 50 is true, it will be a loss 10 yuan. Since 50 is fake, you should pay 60 (the intermediate process is totally unnecessary).