1)-1/4+3/5=-5/20+12/20=7/20
2)9/11+(-2/5)=9/11-2/5=45/55-22/55=23/11
3)-20/21+3/7=-20/21+9/21=-11/21
4)-8 9/14+3 3/7=-121/14+24/7=-121/14+48/14=-73/14=-5 3/14
5)7 5/12+(-3 7/24)=89/12-3 7/24=89/12-79/24=178/24-79/24=99/24=33/8
6)-6 11/12 + (-8 13/18)=-6 11/12- 8 13/18=-(6+8)+(11/12+13/18)=-14+59/36=-14+1 23/36=-15 23/36
7)-5 12/35+10=-187/35+10=163/35=4 23/35