[tex]\it x+y+y+z+x+z=700+500+800 \Rightarrow 2(x+y+z)=2000|_{:2} \Rightarrow\\ \\ \\ \Rightarrow x+y+z=1000\ \ \ (*)\\ \\ \\ x+y=700 \stackrel{(*)}{\Longrightarrow} 700+z=1000\Big|_{-700} \Rightarrow z=300\\ \\ \\ y+z=500 \stackrel{(*)}{\Longrightarrow} x+500=1000\Big|_{-500} \Rightarrow x=500\\ \\ \\ x+z=800 \stackrel{(*)}{\Longrightarrow} 500+y=1000\Big|_{-800} \Rightarrow y=200[/tex]
