First pass: put group A and group B on both sides of the balance. If the weight is the same, the abnormal ball is in group C, otherwise it is in group A and group B;
Discuss separately: (1) If the abnormal ball is in Group C (that is, A is as heavy as B), then
Second time: select three balls 1, 2,3 from group A as standard balls and put them on the left side of the balance, and select three balls 9 from group C, 10,1,and put them on the right side of the balance. If it is balanced, the abnormal sphere is12; If it is unbalanced, the abnormal ball is one of 9, 10, 1 1, and we can know whether the abnormal ball is heavier or lighter than the standard ball.
Third time: put the No.9 ball and 10 ball on the right side of the balance respectively. If it is balanced, the abnormal ball is 65438+01; If it is not balanced, the abnormal ball can be selected according to the weight comparison between the abnormal ball and the standard ball.
(2) If the abnormal ball is in Group A and Group B (that is, A and B are not of the same weight), then Group C is the standard ball, assuming that A is heavier than B, then
Second time: put the balls of 1, 2, 3 and 5 on the left side of the balance, and put the balls of 6, 9, 10,1on the right side. If it is balanced, it means that the abnormal ball must be 4, 7 and 8, and it must be lighter than the standard ball. You can pick out the No.7 and No.8 balls by comparing their weights for the last time. If it is not balanced (it must be the weight on the left), it means that the abnormal ball is in the 1, 2, 3 ball in Group A, and the abnormal ball must be heavier than the standard ball, so any two balls in 1, 2, 3 ball can be picked out after the last comparison.