帮我做下这三个题：871-87.21+56又21分之19-12.79+43又21分之2=

=-9.2+7.4+46/5-32/5-4-3
=-9又1/5+7又2/5+46/5-32/5-20/5-15/5
=-46/5+37/5+46/5-32/5-20/5-15/5
=(-46+37+46-32-20-15)/5
=-30/5
=-6
=18-6/(-2)*100/9
=18+100/3
=154/3871那个帮我做啊=871-(87.21+12.79)+(56又21分之19+43又21分之2)=871-100+100=871