7/10-(3/4-2/5)=14/20-(15/20-8/20)=14/20-7/20=7/20
我写给你计算过程:7/10-(3/4-2/5)=7/10-3/4+2/5=(7/10+2/5)-3/4=(7/10+4/10)-3/4=11/10-3/4=22/20-15/20=7/20
7/10-(3/4-2/5)=7/10-3/4+2/5=14/20-15/20+8/20=14/20+8/20-15/20=22/20-15/20=7/20