x[1-(90-x)×1.6%-60%]=12
x(0.4-90×1.6%+x×1.6%)=12
x(0.4-90×1.6%+x×1.6%)=12
-1.04x+0.016x²=12
x²-65x-750=0
(x-75)(x+10)=0
x=75或 x=-10
x[1-(90-x)×1.6%-60%]=12
x(0.4-90×1.6%+x×1.6%)=12
x(0.4-90×1.6%+x×1.6%)=12
-1.04x+0.016x²=12
x²-65x-750=0
(x-75)(x+10)=0
x=75或 x=-10