（1-80％）x+（1-75％）（280-x）=640.2x+0.25X(280-x)=640.2x+70-0.25x=640.25x-0.2x=70-640.05x=6x=6/0.05x=120(请采纳）
原式：0.2x+0.25（280-x）=64 0.2x+70-0.25x=64 -0.2x=-8 x=40
好简单
