A book store sold 30 books If the ratio of books to bookmarks sold was 10:1 how many bookmarks did the store sell?

The ratio of books to bookmarks is 10:1 (1 bookmark per each 10 books) and the store sold 30 books. If we want to make the left side of the ratio the same as amount of sold books, we need to multiply it by 3 (10 * 3 = 30). If we multiply the left side by 3, we have to do the same with the right side and we'll get the result of sold bookmarks (1 * 3 = 3).

In short:

10:1 = 30:3

Answer: The store sold 3 bookmarks.