[tex]a+b=300 \\ b+c=500 \\ a+c=400=>adunam\ \ cele\ \ trei\ \ relatii \\ ======= \\ 2a+2b+2c=1200/:2=>\boxed{\boxed{a+b+c=600}} \\ \\ \\ 300+c=600 \\ c=600-300 \\ \\\boxed{ c=300} \\ \\ a+500=600 \\ a=600-500 \\ \\ \boxed{a=100} \\ \\ a+b=300 \\ 100+b=300 \\ \\ \boxed{b=200}[/tex]
