\[\boxed{\mathbf{411}.}\]
\[\frac{x\ :3}{21\ :3} = \frac{5}{y}\]
\[\frac{x\ :3}{7} = \frac{5}{y}\]
\[y = 7.\]
\[x\ :3 = 5\]
\[x = 5 \cdot 3\]
\[x = 15.\]
\[Ответ:x = 15;\ \ y = 7.\]