The factor is a number when multiplied by another gives a specific number. The greatest common factor of 2 numbers is the bigger number which both numbers can be divide by to give whole numbers. 20/2 = 10 and 36/2 = 12, so 2 is a common factor of 20 and 36. 20/4 = 5 and 36/4 = 9, so 4 is a common factor of 20 and 36. 20 and 36 are pair numbers so they can't be divided by 5. 20/6 gives something like 3.3 so it's not a whole number.
Since both 5 and 6 are rejected, and 4>2, this means that the Greater common factor of 20 and 36 is 4.
