20,9:5,5;1 1/21:2/7;2 4/17:10/17

20,9:5,5= 209/55 = 3_8/10 =3,8 = 3_4/5

1 1/21:2/7 = 11/21 * 7/2 = (11*7) / (21*2) = 11/6 = 1_5/6

2 4/17:10/17 = 38/17 * 17/10 = (38*17) / (17*10) = 38/10 = 3,8