由x+y=300得y=300-x 代人30(1-20%)(x+20%x)+50y=12880得 30(1-20%)(x+20%x)+50(300-x)=12880 30*0.8*x*1.2+15000-50x=12880 15000-12880=50x-28.8x 2120=21.2x x=100