Answer :
1). List all the factors of the first number.
2). List all the factors of the second number.
3). Make a "short list", of the factors that are on both lists.
These are the 'common' factors.
4). Find the biggest number on the short list.
That's the 'greatest common factor' of the original two numbers.
Divide the number by prime numbers that go into all 3 or 2 numbers. Then you multiply the numbers you used to divide by are multiplied together. Ex: 2I_18_30_24_
3I__9__15__12__
2x3=6
6 is your Greatest Common Factor