The so-called unit "1" means that in a practical problem, there are many quantities, but the actual quantities of some of these quantities are not clear. In order to solve the problem, the "1" unit is introduced. In this way, we can compare the remaining quantity with this reference quantity and express it in this unit "1" (reference quantity). Because the basic units of these reference quantities are the same, we can do related calculations about addition, subtraction, multiplication and division that meet the meaning of the question.