1/4+7/10-2/5=0.25+0.7-0.4=0.55
1/4+7/10-2/5=0.25+(0.7-0.4)=0.25+0.3=0.55
原式=7/10-4/10+1/4=6/20+5/20=11/20
1/4+7/10-2/5=5/20+14/20-8/20=25/20-8/20=17/20