Suppose the customer didn't pay directly, but the fact is that he didn't pay.

In the first step, 30 yuan's shoes were sent to customers, which damaged 30 yuan.

Step 2: Give the customer 30 yuan, and then hurt 30 yuan. .

The third step, the neighbor first gave it to Master Wang, 50 yuan, and later Master Wang gave it to the neighbor, 50 yuan. There is no money gain or loss between neighbors and Master Wang.

Total loss: 30+30=60 yuan.

Add operation

In the formula with brackets, the inside of brackets should be counted first, then the inside of brackets, and finally the outside of brackets.

1, elementary arithmetic order: when operating at the same level, it is calculated from left to right in turn; Two-stage operation, first multiply and then divide, then add and subtract.

When there are brackets, count the inside of brackets first, and then count the outside of brackets; When there are multiple brackets, count the brackets first, then the brackets inside, then the braces inside, and finally the brackets outside.

2. Multiplication is a simple operation of addition and division is a simple operation of subtraction. Subtraction and addition are reciprocal operations, and division and multiplication are reciprocal operations.

When several addends are added, the positions of addends can be exchanged at will; Or add a few addends first and then add them with other addends, and the sum remains the same.