A Greatest Common Factor (or GCF) is when you list all the factors of the numbers you have and you find the biggest factor of BOTH the numbers. They MUST be the same.
Ex) 28 and 30.
You list al the factors of 28 and 30.
28- 1,2,4,7,14,28.
30- 1, 2, 3, 5, 6, 10, 15, 30.
Both numbers have all their factors.
What is the factor that both numbers have that is the biggest? 2.
So your Greatest Common Factor (GCF) is 2.