①令x/2=y/4=z/7=k
所以x=2k,y=4k,z=7k
所以x：y：z=2:4:7
②2x+y+3z=58
即4k+4k+21k=58
29k=58
k=2
x=2k=4
y=4k=8
z=7k=14
③(x+y)/(x+z)
=(2k+4k)/(2k+7k)
=6/9
=2/3