\[\boxed{\mathbf{24.}}\]
\[8\ :2 = 4\]
\[12\ :2 = 6\]
\[18\ :2 = 9\]
\[6\ :3 = 2\]
\[12\ :3 = 4\]
\[15\ :3 = 5\]
\[8\ :4 = 2\]
\[16\ :4 = 4\]
\[20\ :4 = 5\]
\[15\ :5 = 3\]
\[12\ :6 = 2\]
\[20\ :5 = 4\]