Question

There are 16 books on a shelf. 7 of these books are new. (a) What is the ratio of new books to used books? (b) What is the ratio of all books to used books?

Answer 1

The answers are
a. 7:9
b. 16:7

Answer 2

7 out of the 16 books are new, so 9 of them are used. 

7:9 is the ratio of new books to old books. 
16:9 is the ratio of all books to used books.