100*x+50*(40-x)=3000
100*x+2000-50x=3000
50X=1000
X=20能再详细点吗？100*x+50*(40-x)=3000100*x+50*40-50*x=3000100*x+2000-50x=3000100*x-50x=3000-200050X=1000X=20