[mx+n(30-x)]/(30-x+x)=[nx+m(20-x)]/(20-x+x)
20(mx-nx+30n)=30(nx-mx+20m)
20(m-n)x+600n=-30(m-n)x+600m
50(m-n)x=600(m-n)
x=600(m-n)/[50(m-n)]=12(千克).
[mx+n(30-x)]/(30-x+x)=[nx+m(20-x)]/(20-x+x)
20(mx-nx+30n)=30(nx-mx+20m)
20(m-n)x+600n=-30(m-n)x+600m
50(m-n)x=600(m-n)
x=600(m-n)/[50(m-n)]=12(千克).