\[x\ рублей - ватрушка;\ \ \]
\[y\ рублей - плюшка.\]
\[\left\{ \begin{matrix} 3x + 5y = 45\ \ \ | \cdot 5 \\ 5x + 3y = 43\ \ \ | \cdot 3 \\ \end{matrix} \right.\ \text{\ \ \ \ \ }\]
\[\ \left\{ \begin{matrix} 15x + 25y = 225 \\ 15x + 9y = 129\ \ \\ \end{matrix} \right.\ \text{\ \ }( - )\]
\[\overline{\ \ \ \ \ 16y = 96\ \ \ \ \ |\ :16}\]
\[\ \ \ \ \ \ \ \ \ y = 6\]
\[\left\{ \begin{matrix} y = 6\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ 3x + 5 \cdot 6 = 45 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} y = 6\ \ \ \ \ \\ 3x = 15 \\ \end{matrix} \right.\ \text{\ \ \ \ \ \ }\]
\[\left\{ \begin{matrix} y = 6 \\ x = 5 \\ \end{matrix} \right.\ \]
\[Ответ:5\ рублей;6\ рублей.\ \]