x*z/(20+x+y)=3; z/(20+x+y/2)=0.45; x*z /(x+20)=9; 请问此方程如何解

令20+x=a,则x=a-20,
∴(a-20)z/(a+y)=3 ①,z/(a+y/2)=0.45 ②,(a-20)z/a=9 ③,
③/①得(a+y)/a=3,即y=2a,
将y=2a代入②,得z=0.9a,
将z=0.9a代入③,得0.9(a-20)=9,∴a=30
故x=10,y=60,z=27.