Explanation:
To find the ratio of new books to all books on the shelf, you add the number of new books and used books together:
New books + Used books = 7 (new books) + 6 (used books) = 13 books in total
So, the ratio of new books to all books is:
New books / All books = 7 (new books) / 13 (total books) = 7/13