By the ratio of two numbers we mean that is a division which results in the same value. It is, if the ratio of the number of books to the bookmarks sold at the same period is 8:2, it means the division of the number of books divided by the bookmarks sold mus be equal to 8:2 = 4.
We already know the number of books, that is 48, but we do not know the number of bookmarks, so we call it x. And we have the following:

To isolate x and find its value, we multiply both sides by x, and divide, also both sides, by 4. It is:

From the presented calculation, we can say that the amount of bookmarks sold during Christmas was 12.