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.
