Ответ:

1)(8 7/12-х):20=7/40

8 7/12-х=7•20/40

8 7/12-х=7/2: х=8 7/12-3 1/2; х=8 7/12-3 6/12;

х= 5 1/12

2) 36:(х+11 4/5)=1 17/19; х+11 4/5=36:1 17/19;

х+11 4/5=36:36/19; х+11 4/5=36•19/36;

х+11 4/5=19; х=19-11 4/5; х=18 5/5-11 4/5; х=7 1/5

3)44:(х-9 3/4)=1 9/13; х-9 3/4=44:1 9/13:

х-9 3/4=44:22/13; х-9 3/4=44•13/22; х-9 3/4=26:

х=26+9 3/4; х =35 3/4