20 8/15 - 11 11/35 =308/15-396/35=
=(308*7-396*3)/105=(2156-1188)/105=
=968/105=9 23/105
7 4/9 - 6 8/9 =6 13/9-6 8/9=5/9
20 8/11 + 2 7/11=23 4/11
14 7/12 - 5 9/12=13 19/12-5 9/12=8 10/12=8 5/6
