Description: Find the biggest number that will divide into the given 2 numbers. This is called the GCF or greatest common factor.No worries, we'll show you how to do it. Created by Sal Khan. And they just say, another way to say this is the GCD, or greatest common divisor, of 20 of 40 is equal to question mark. And greatest common divisor sounds like a very fancy term, but it's really just saying, what is the largest number that is divisible into both 20 and 40? One way, you could literally list all of the factors of each of these numbers and figure out which one is greater or what is the largest factor of both.*Teacher Advisor is 100% free.