The greatest common factor (GCF) is the biggest number that can multiply into a set of numbers.
List the factors for 8 and 28:
Factors of 8: {1,2,4,8}
Factors of 28: {1,2,4,7,14,28}
There are three common factors between 8 and 28:
1, 2, 4
The biggest number of these three is 4, so the GCF for 8 and 28 will be 4.