The greatest common factor is 4.
Solution by Factorization:
The factors of 20 are: 1, 2, 4, 5, 10, 20
The factors of 80 are: 1, 2, 4, 5, 8, 10, 16, 20, 40, 80
The factors of 128 are: 1, 2, 4, 8, 16, 32, 64, 128
Then the greatest common factor is 4.
