First weighing: 1, 2,3,4 to 5,6,7,8.

If they are equal, the difference is 9, 10, 1 1, 12, 13.

Second weighing: 9 10, 1 1 to 1, 2,3.

If they are equal, the differences are 12 and 13.

Third weighing: 12 to 1.

If they are equal, the answer is13;

If not, the answer is 12.

If 9, 10, 1 1 and 1, 2,3 are different, the difference is 9, 10,1. At this time, it can be judged whether one side of 9 10, 1 1 is light or heavy.

Third weighing: 9 pairs 10.

If they are equal, the answer is 1 1.

If they are not equal, and there is one side weight in 9, 10, 1 1, the weights in 9 and 10 are different;

If one side of 9, 10, 1 1 is light, then the light side of 9 and 10 is different.

If 1, 2, 3, 4 and 5, 6, 7, 8 are different, then the difference is between these eight balls.

Second weighing: 1, 2, 3, 5, 6 to 9, 10,1,12, 13.

If equal: the difference is in 4, 7, 8.

The third weighing: 7 to 8.

If equal: 4 is different.

If not, 5, 6, 7 and 8 are heavier, and the answer is the heavier of 7 and 8; On the contrary, it is the light one.

If 1, 2,3,5,6 is different from 9, 10, 1, 12, 13, the difference is1,2,3. Moreover, 1, 2, 3, 5, 6 is heavier, and the difference is on the heavier side of 1, 2, 3, 4 and 5, 6, 7, 8. (and vice versa)

Suppose it is 1, 2, 3.

Third weighing: 1 to 2.

Equal, it is 3;

No, that's heavy.

Suppose it's 5, 6.

Third weighing: 1 to 5.

Equal, it is 6;

No, it is five o'clock.