At least three times, put the 10 bag on the left and the 10 bag on the right once. For the second time, divide the light 10 bag into 5 bags on the left and 5 bags on the right, and divide the light 5 bag into 2 bags, 2 bags, 1 bag, 2 bags on the left and 2 bags on the right. If the two bags weigh the same this time, it can be concluded that the remaining bags are defective, which is the least frequent.