[tex]\it \dfrac{a}{b} = \dfrac{3}{5} \stackrel{derivare}{ \Longrightarrow} \dfrac{a}{3} = \dfrac{b}{5} =k \Rightarrow \begin{cases} \it a=3k\\ \\
\it b=5k \end{cases} \ \ (*)
\\ \\ \\
\dfrac{3a+b}{a+5b} \ \stackrel{(*)}{=} \ \dfrac{3\cdot3k+5k}{3k+5\cdot5k} = \dfrac{\ 14k^{(k}}{28k } = \dfrac{\ 14^{(14}}{28} = \dfrac{1}{2}[/tex]