[tex]\it \dfrac{5a+2b}{7a+2b}=\dfrac{4}{5} \Rightarrow \dfrac{5a+2b}{7a+2b-5a-2b}=\dfrac{4}{5-4} \Rightarrow \dfrac{5a+2b}{2a}=4 \Rightarrow \\ \\ \\ \Rightarrow 5a+2b=8a|_{-5a} \Rightarrow 2b=3a|_{:2} \Rightarrow b=1,5a\ \ \ \ \ \ (*)\\ \\ \\ \dfrac{2a-b}{a+b}\ \stackrel{(*)}{=}\ \dfrac{2a-1,5a}{a+1,5a}=\dfrac{0,5a}{2,5a}= \dfrac{^{2)}0,5}{\ 2,5}=\dfrac{1}{5}[/tex]
