1. You choose 2 or more numbers (lets say 8 and 42)
2. You find ALL of the factors of the two numbers(8:1,2,4,8; 42: 1, 2, 3, 6, 7, 14, 21, 42)
3. You find equivalent, or the same factors of both numbers (8,12:1, 2)
4. The you se the last number on the common factors. he last number (which is 2) is the GCF
Hope this helps
