According to the previous quantity relationship, the unit price and quantity cannot be interchanged, but now the unit price and quantity can be interchanged.

Originally, because unit price and quantity are two different quantities, unit price refers to how much a book costs, and quantity refers to the number of books to be bought. According to the quantitative relationship, how much should I buy the first book of fifth grade mathematics? It should be: unit price x quantity = total price. Now it is ok to find the total price: quantity x unit price = total price, and the position can be exchanged.