Example: 30 and 42
1, 2, 3, 5, 6, 10, 15, 30
1, 2, 3, 6, 7, 14, 21, 42
The GCF is 6.
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1,2,3,4,6,8,12,16,24,48
They are: 1, 2, 11 and 22
1). List all of the factors of 16. 2). List all of the factors of 20. 3). List all of the factors of 28. 4). Make a short list, comprised of any numbers that are on all three lists. These are the common factors of 16, 20, and 28. 5). Find the greatest number on the short list. That's the greatest common factor of 16, 20, and 28. If it is not 4, then an error has crept into your work, and you are doomed to return to Step-1 and eternally repeat the process until you get 4 .
List the factors of each number. The largest number that appears on both lists is the GCF.
1, 3, 5, 9, 15, 45.