方法一1200×30%=360(吨)1200-360=840(吨)840×40%=336(吨)1200-360一336=504(吨)方法二1200x[1-30%-(1-30%)×40%]=1200×42%=504(吨)
[（1200*0.7）*0.6]
1200x(1-30%-40%)=1200x30%=360(吨)
