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=70000
900x+77760-972x=70000
900x-972x=70000-77760
-72x=-7760
x=970/9