\[x\ рублей - марка;\ \ \]
\[y\ рублей - конверт.\]
\[\left\{ \begin{matrix} 3x + 5y = 36\ \ \ | \cdot 4 \\ 4x + 2y = 24\ \ \ | \cdot 3 \\ \end{matrix} \right.\ \]
\[\left\{ \begin{matrix} 12x + 20y = 156 \\ 12x + 6y = 72\ \ \ \ \ \\ \end{matrix} \right.\ \text{\ \ \ }( - )\]
\[\overline{\ \ \ \ \ \ 14y = 84\ \ \ \ \ \ \ \ }\]
\[\ \ \ \ \ \ \ \ \ \ y = 6\]
\[\left\{ \begin{matrix} y = 6\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ 3x + 5 \cdot 6 = 39 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} y = 6\ \ \ \\ 3x = 9 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} y = 6 \\ x = 3 \\ \end{matrix} \right.\ \]
\[Ответ:6\ рублей;\ \ 3\ рубля.\ \]