The factors of 9 are 1, 3 and 9. A factor is a number that divides into another number evenly. When a person divides 9 by the numbers 9, 3 and 1, it is easy to see that they are factors.
To find factors of larger numbers, it is useful to know the different divisibility tests. For example, there are tests to see if a given number is divisible by 2, 3, 4, 5, 6, 9 and 10. The divisibility rule for 3 states that a number is divisible by 3 if the sum of its digits also can be divided by 3. By using this rule, it is evident that a number like 27, for which the sum of the digits 2 and 7 is equal to 9, is also divisible by 3.