Common factors of 16a and 20ab.

Answer:[tex]\large\boxed{Common\ factors:\ 1,\ 2,\ 4,\ a,\ 2a,\ 4a}[/tex]Step-by-step explanation:[tex]16a=2\cdot2\cdot2\cdot2\cdot a\\Factors:\boxed1,\ \boxed2,\ \boxed4,\ 8,\ 16,\ \boxed{a},\ \boxed{2a},\ \boxed{4a},\ 8a,\ 16a\\\\20ab=2\cdot2\cdot5\cdot a\cdot b\\Factors:\boxed1,\ \boxed2,\ \boxed4,\ 5,\ 10,\ 20,\ \boxed{a},\ \boxed{2a},\ \boxed{4a},\ 5a,\ 10a,\ 20a,\\b,\ 2b,\ 4b,\ 5b,\ 10b,\ 20b,\ 2ab,\ 4ab,\ 5ab,\ 10ab,\ 20ab\\\\Common\ factors:\ 1,\ 2,\ 4,\ a,\ 2a,\ 4a[/tex]