What is the GCF of 28 20 and 40?

To find the GCF, you first need to break the numbers down into their prime factors:

28 = 2x2x7

20 = 2x2x5

40 = 2x2x2x5

The next step is to find any common prime factors. In this case, all three numbers have two 2s as prime factors. Multiply these and you get 4. Therefore the GCF of 28, 20 and 40 is 4.