①x+2y-4z＝0
②3x+y-z＝0
①-2②
x-6x-4z+2z=0
5x=2z 代入①
z=5x/2
x+2y-10x=0
2y=9x
y=9x/2
x:y:z=1:9/2:5/2=2:9:5