(X-50)[800-20(X-60)]=12000(X-50)[800-20(X-50-10)]=12000800*(x-50)-20(x-50)^2+200(x-50)=12000-20(x-50)^2+1000(x-50)-12000=0(x-50)^2-50(x-50)+600=0(x-50-30)(x-50-20)=0x=80 或x=70