This is a simple question of proportion. If you want to know the ratio between the number of books borrowed and the number of new books, the answer is the ratio between the number of books borrowed and the number of new books. This is also the way to solve all the proportion problems, that is, divide the required quantity by the total, and the answer is proportion.

So the library has 100 new books, and 42 books have been borrowed, so the percentage of borrowing books should be 42%, which is 2 1/50 after simplification.