How you can find the common factors and the greatest common factor of two numbers

One way is to create a factor tree for each number. Then see which factors they have in common.

Example:

12 30

^ ^

2 6 3 10

^ ^

2 3 2 5

The factors are the underlined numbers above:

12: 2 x 2 x 3

30: 2 x 3 x 5

The common factors are: 2, 3, and 6 why 6? because 2 x 3 = 6

The greatest common factor is: 6