What is the highest common factor of 120 and 405?

It's 15 because the factors of 120 are... (down below) and the factors of 405 are... (down below) and the only factors that are the same are 1, 3, 5, and 15 so it has to be 15.

120: 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120

405: 1, 3, 5, 15, 27, 81, 135, 405

SAME#: 1, 3, 5, 15