12乘400乘0.75x+12乘360（80-x）乘0.9=280000 3600x+12×36×(80-x)×9=280000两边都除以4,得900x+3×36×(80-x)×9=70000900x+77760-972x=70000900x-972x=70000-77760-72x=-7760x=970/9
12乘400乘0.75x+12乘360（80-x）乘0.9=280000
12乘400乘0.75x+12乘360（80-x）乘0.9=280000 3600x+12×36×(80-x)×9=280000两边都除以4,得900x+3×36×(80-x)×9=70000900x+77760-972x=70000900x-972x=70000-77760-72x=-7760x=970/9