Question

There are 15 books on a shelf. of these books 7 are new.

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

Answer 1

a) 8:7
b) 8:15

Answer 2

To get the number of used books. You have to find the number of books left on the shelf.
So:
\[15-7=?\]

Answer 3

used books:new books = 15
already know new books = 7
therefore 15-7=8
thus ratio = 8:7

Answer 4

the ratio of used books:all books = 8:15