It is equivalent to spending 1 book to buy three books, and each cost is equivalent to one-third of the original price. Such as: 6 yuan 1 bottle of beer, buy 1 get 2 free. The guest bought two bottles of beer, sent four bottles, and spent 12 yuan got six bottles of beer, which cost only one third of the original price, that is, one bottle in 2 yuan.

Mathematics is a subject that studies concepts such as quantity, structure, change and space with symbolic language. The discipline of explaining relations (quantitative relations, structural relations, changes before and after) in language is produced by counting, calculating, measuring and observing the shape and movement of objects by using abstract and logical reasoning. Math problems include: oral calculation problems, fill-in-the-blank problems, judgment problems, overview problems, proof problems, calculation problems and picture-looking problems.