Whole numbers that are multiplied together to make another number are known as factors. When finding a factor of a number, that number and the number 1 are the most obvious factors, as every number can be divided or multiplied by 1, as well as be divided by itself to make 1. While these are important factors, when someone is finding factors, they're also looking for all of the other numbers that can be multiplied together to make that number.
To find a factor by division, figure out what numbers can be multiplied together to make that number. For instance, with the number 12, if you divide it by 2 you get 6, meaning 2 and 6 are factors. As well, divide 12 by 3 and get 4, so that means 3 and 4 are factors. If you divide 12 by 5, you don't get a whole number, thus 5 is not a factor of 12. After trying each number equal to or smaller than 12, you see the factors of 12 are: 1, 2, 3, 4, 6, and 12.
You can also multiply numbers together to learn about factors. The numbers 4 and 5, when multiplied together, make 20, meaning that 4 and 5 are factors of 20. However, it is important to remember that 4 and 5 are not the only factors of 20 and that 4 and 5 are factors of other numbers too.
Not every number has a factor other than 1 and itself. Numbers that can only be divided by itself and 1 are called prime numbers. No whole numbers, other than itself and 1, can be multiplied together to make these numbers. Some examples include 2, 3, 5, 7, 11, 13, 17 and 19.