Question

Common factor of 16a and 20ab

Answer 1

The common factor of 16 is  1, 2, 4, 5, 8, 10, 20 and 40.
the common factor of 20 is 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120. 

Answer 2

If you are asking for a common factor of 16 AND 20 the answer can be 80

Hope this helped :)