\[\boxed{\mathbf{565.}\mathbf{\ }}\]
\[\frac{6,4 + y}{2} = 8,5\ \ \ \ \ \ \]
\[6,4 + y = 8,5 \cdot 2\]
\[6,4 + y = 17\]
\[y = 17 - 6,4\]
\[y = 10,6.\]
\[Ответ:\ \ 10,6.\]