x:y:z=2：3：4,
则x/2=y/3=z/4
令x/2=y/3=z/4=k
x=2k,y=3k,z=4k
x+y+z=18
2k+3k+4k=18
9k=18
k=2
x=2k=4
y=3k=6
z=4k=8