The sum, difference, product and quotient of the number of books lent and returned every day are not necessarily the same, so the library has a certain number of books, and the number of books lent and returned every day is disproportionate.

To judge whether two related quantities are out of proportion and what proportion they are, it depends on whether the corresponding ratio of the two quantities is certain or whether the corresponding product is certain. If the proportion is certain, it is proportional; If the product is certain, it is inversely proportional; If other quantities are certain or the product sum ratio is not certain, it is out of proportion.