tatsach 3) -2 5/7+3=2/7
2/7≠4/7

4) -21/72+20/72=-1/72
-1/72≠1/72

5) =2 9/11-3 7/22=31/11-73/22=-42/22=-1 10/11
-1 10/11≠1 3/22

10) 9-11=-2

17)=-16/40+15/40=-1/40
-1/40≠1/40

20)=7/14-12/14=-5/14
-5/14≠-6/14