what is the GCF of 30 and 75?
Method I: List all of the factors of the two numbers and find the largest number the lists have in common.
Thus the GCF is 15.
Method II: A more general method, and one that works better with many factors or problems involving variables.
First we write each number as a product of prime factors.
We then circle any number that appears in both lists -- in this case we have a 3 and a 5 in both lists. The GCF will be the product of these common factors, or GCF=3*5=15.