x+y+z=142 x/28+y/30+z/35=4.5 z/28+y/30+x/35=4.7 怎么解.

(z/28+y/30+x/35)-(x/28+y/30+z/35)=(1/28-1/35)(z-x)=(z-x)/140=0.2
z-x=28
(z/28+y/30+x/35)+(x/28+y/30+z/35)=(1/28+1/35)(x+z)+y/15
=9/140*(2x+28))+y/15=9x/70+y/15+1.8=9.2
9x/70+y/15=9.2-1.8=7.4
27x+14y=1554
y=(1554-27x)/14=111-27x/14
x+y+z=x+111-27x/14+x+28=x/14+139=142
x=42
y=30
z=70