For the first time, put 1-4 on the left and 5-8 on the right.

1. If the balance is balanced, the bad ball is 9- 12.

The second time, put 1-3 on the left and 9- 1 1 on the right.

1. If the weight is appropriate, the bad ball is 9- 1 1, and the bad ball is heavier.

Put No.9 on the left and 10 on the right for the third time.

1. If the weight is appropriate, 10 is a bad ball, which is heavier than the standard ball;

2. If it is balanced, the number 1 1 is a bad ball, which is heavier than the standard ball;

3. If the left ball is heavy, No.9 is a bad ball, which is heavier than the standard ball.

2. If it is balanced, the bad ball is 12.

For the third time, put 1 on the left and 12 on the right.

1. If the weight is appropriate, 12 is a bad ball, which is heavier than the standard ball;

2. It is impossible to balance at this time;

3. If the left ball is heavy, the number 12 is a bad ball, which is lighter than the standard ball.

3. If the left is heavy, the bad ball is 9- 1 1, and the bad ball is lighter.

Put No.9 on the left and 10 on the right for the third time.

1. If the weight is appropriate, the No.9 ball is a bad ball, which is lighter than the standard ball;

2. If it is a balance ball, the number 1 1 is a bad ball, which is lighter than the standard ball;

3. If the left ball is heavy, 10 is a bad ball, which is lighter than the standard ball.

2. If the left is heavy, the bad ball is in 1-8.

Remove No.2-4 for the second time, move No.6-8 from right to left, and put No.9-11.

It's on the right. That is to say, put 1, 6, 7, 8 on the left and 5, 9, 10,1/on the right.

1. If the right ball is heavy, the bad ball will be taken to the left 6-8, which is lighter than the standard ball.

For the third time, put No.6 on the left and No.7 on the right.

1. If the weight is appropriate, No.6 is a bad ball, which is lighter than the standard ball;

2. If it is a balance ball, the No.8 ball is a bad ball, which is lighter than the standard ball;

3. If the left is heavy, No.7 is a bad ball, which is lighter than the standard ball.

2. If it is balanced, the bad ball is heavier than the standard ball in the number 2-4 that has been removed.

Put No.2 on the left and No.3 on the right for the third time.

1. If the weight is appropriate, the No.3 ball is a bad ball, which is heavier than the standard ball;

2. If it is balanced, No.4 is a bad ball, which is heavier than the standard ball;

3. If the left ball is heavy, No.2 is a bad ball, which is heavier than the standard ball.

3. If the left is heavy, the bad ball is at the untouched number 1, 5. If it is 1,

It is heavier than the standard ball; If it's a No.5 ball, it's lighter than the standard ball.

Put 1 on the left and No.2 on the right for the third time.

1. It can't be right this time.

2. If it is balanced, the No.5 ball is a bad ball and lighter than the standard ball;

3. If the left ball is heavy, then the number 1 is a bad ball, which is heavier than the standard ball.

3. If the right hand is heavy, the situation is contrary to 2, and the same idea is the solution.

Thirteen ping-pong balls all have the same characteristics, except one with abnormal weight. Now it is required to weigh the ball three times with a balance without weight to find out the ball with abnormal weight.

Note: The weight is abnormal, and there is no clear weight.

The answer is as follows: number the ball first 1- 13,

For the first time, put 1-4 on the left and 5-8 on the right.

1. If it is balanced, the bad ball is 9- 13.

The second time, put 1-3 on the left and 9- 1 1 on the right.

1. If the weight is appropriate, the bad ball is 9- 1 1, and the bad ball is heavier.

Put No.9 on the left and 10 on the right for the third time.

1. If the weight is appropriate, 10 is a bad ball, which is heavier than the standard ball;

2. If it is balanced, the number 1 1 is a bad ball, which is heavier than the standard ball;

3. If the left ball is heavy, No.9 is a bad ball, which is heavier than the standard ball.

2. If it is balanced, the bad balls are 12 and 13.

For the third time, put 1 on the left and 12 on the right.

1. If the weight is appropriate, 12 is a bad ball, which is heavier than the standard ball;

2. If it is balanced, the number 13 is a bad ball, and three chances have been used up so far, but the number 13 has not been weighed;

3. If the left ball is heavy, the number 12 is a bad ball, which is lighter than the standard ball.

3. If the left is heavy, the bad ball is 9- 1 1, and the bad ball is lighter.

Put No.9 on the left and 10 on the right for the third time.

1. If the weight is appropriate, the No.9 ball is a bad ball, which is lighter than the standard ball;

2. If it is a balance ball, the number 1 1 is a bad ball, which is lighter than the standard ball;

3. If the left ball is heavy, 10 is a bad ball, which is lighter than the standard ball.

2. If it is unbalanced, refer to steps 2 and 3 of 12 ball to answer, because the problem at this time will be transformed into the same problem, that is, finding the abnormal ball from 8 balls twice.