Master Wang lost 60 yuan.

The following is the analysis:

Fa 1

Suppose Master Wang has 100, a pair of shoes, 70 yuan after the purchase, and Master Wang has 120 yuan after the customer gives money, and the change is 90 yuan.

After losing to neighbor 50 yuan, only 40 yuan 100-40=60 (yuan) is left.

Method 2 (Hypothetical Method)

When the customer gives real money, Master Wang loses 10 yuan, and when he gives fake money, he loses 50+ 10 = 60 yuan.

Law 3 (Conservation of Money)

A * * * three people, neighbors don't make money, customers don't spend money to earn a pair of shoes 30 yuan and 30 yuan change ***60 yuan. According to the conservation of money, the money earned by customers must come from Master Wang, so Master Wang lost 60 yuan.

Method 4

(essentially the same as 2, this is a step-by-step calculation)

Loss of selling shoes 10 yuan, loss of counterfeit banknotes 50, total loss 60.

Summary:

The crux of the problem is that the process of finding money is confusing information, which has no effect on profit and loss in essence. After we understand this doubt through abstraction, the above four solutions are just expressing the problem.

Hope to adopt!