[tex]\it 2a+2b=70 \Rightarrow 2(a+b)=70\Big|_{:2} \Rightarrow a+b=35 \Rightarrow a=35-b\ \ \ \ (1)\\ \\ 2(a-b)=10\Big|_{:2} \Rightarrow a-b=5 \Rightarrow a=5+b\ \ \ \ (2)\\ \\ \\ (1),\ (2) \Rightarrow 5+b=35-b \Rightarrow b+b=35-5 \Rightarrow 2b=30|_{:2} \Rightarrow b=15[/tex]
