The fraction 20/21 is already in lowest terms.
21/20 is in its simplest form.
It is: 20/84 = 5/21 in its simplest form
20/210 = 2/21
21/60 = 7/20
21/20
20/21
20/21
42/20 = 21/10 = 21/10
21/80 is already in its simplest form
21/2 or 101/2
40/42 = 20/21