[tex]\displaystyle a+b=700;~b+c=500;~a+c=800 \\ \\ a+b+b+c+a+c=700+500+800 \\ \\ 2a+2b+2c=2000 \\ \\ 2(a+b+c)=2000 \\ \\a+b+c= \frac{2000}{2} \Rightarrow a+b+c=1000 \\ \\ a+500=1000 \Rightarrow a=1000-500 \Rightarrow \boxed{a=500} \\ \\ b+800=1000 \Rightarrow b=1000-800 \Rightarrow \boxed{b=200} \\ \\ 700+c=1000 \Rightarrow c=1000-700 \Rightarrow \boxed{c=300} \\ \\ V : a+b+c=1000 \Rightarrow 500+200+300=1000[/tex]
