Cookies are also used to remember which page of the book you last viewed, so that when you revisit the site you automatically return to the last page you visited.
The numbers 2 and 13 can be added to give 15, but in that case they are not being used as factors. Factors are multiplied together, not added together. The numbers 2 and 13 would be the factors of 26, since 2 × 13 = 26.
To find the factors of 15, find two numbers which, when multiplied together, give 15 as a product. Now return to page 14 and pick them out.