可以把分数化成小数计算:[1-(1/4+2/5)]÷3.5=[1-(0.25+0.4)]÷3.5=(1-0.65)÷3.5=0.35÷3.5=0.1
[1-(1/4+2/5)]÷3.5=(1-13/20)÷(7/2)=7/20X2/7=1/10=10分之1
[1-(1/2+1/10)×5/9]÷4/9简便计算[1-(1/2+1/10)×5/9]÷4/9=(1-3/10*5/9)÷4/9=5/6*9/4=15/8