There are

13 books on a shelf.

8 of these books are new.

(a) What is the ratio of used books to new books?

(b) What is the ratio of used books to all books?

a). 5:8

b). 5:13

Step-by-step explanation:

a) Since 8 are new and there are 13 in total.

You subtract 13-8 which is 5

So now you have 5 used books

They gave you the new book count, which is 8, so it is 5:8

b). You already know the amount of used books: 5 and there are 13 books in all,

so it is 5:13

The ratio of used books to new books is 5:8, since 13-8=5.

The ratio of used books to all books is 5:13, since there are 13 total books and 5 used books.