What is the greatest common factor of 20 and 12?

The greatest common factor of 20 and 12 is 4. This is because 4 is the largest number that divides both 20 and 12 evenly without leaving a remainder. To find the greatest common factor, you can list out all the factors of each number and find the largest factor they have in common. In this case, the factors of 20 are 1, 2, 4, 5, 10, and 20, and the factors of 12 are 1, 2, 3, 4, 6, and 12. The largest factor they have in common is 4.