[(100-x)*10%]*(1-x/100)=0.4,
问题描述：
[(100-x)*10%]*(1-x/100)=0.4,
答
两边同时乘以1000得
[(100-x)*10%*10]*[(1-x/100)*100]=0.4*1000
[(100-x)*1]*(100-x)=400
(100-x)*(100-x)=400
100-x=20
x=80