解如下图所示
3/10-(1/4-2/9)= 3/10-1/4+2/9【通分】=6/20-5/20+2/9=1/20+2/9【通分】=9/180+40/180=49/180
原式=3/10-1/4+2/9=(54-45+40)/180=49/180.
(5/28+2/9)-(1/3+1/4)=(2/9-1/3)+(5/28-1/4)=-1/9-1/14=-23/126