Fourth-grade mathematics: knowledge stores carry out promotional activities. For every book in 25 yuan, buy 5 books and get 1 book. If you buy 40 books, how much will it cost? Every book is cheap.
If you buy five books as a group, which is actually: 6x6+4=40, then you buy six groups and give away six books, so you can buy 40 books only by paying for 34 books, so * * * spent: 34x25=850 yuan is actually cheaper: 25-850/40=3.75 yuan.