Method and formula for finding defective products: divide the items to be tested into three parts as evenly as possible; If not, let the two be equal, and the difference between the third part and the two parts shall not exceed 1, so that the defective products can be found in the least number of times.

Finding defective products is the main type of primary school olympiad. Now in school textbooks, this question type appears in the "wide angle of mathematics". The basic question type is that one of several parts is different from other parts, and this part is lighter or heavier than other parts. With the weightless balance, at least defective products can be found many times. The total number of general parts is divided into three parts on average. If it can't be divided equally, it is divided into three forms: A, A and B. A is more than b 1 or less than b 1, and cannot be greater than 2 and the latter is less than 2.