In order to solve this, we start by listing the factors of 20 and 40, like this:
factors of 20: 1, 2, 4, 5, 10, 20
factors of 40: 1, 2, 4, 5, 8, 10, 20, 40
As you can see, 20 is present in both lists, this number is also the greatest factor that they have in common then the greater common factor of 20 and 40 is 20