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.