\[\boxed{\mathbf{6.}}\]
\[10\ :2 = 5\]
\[14\ :2 = 7\]
\[16\ :2 = 8\]
\[9\ :3 = 3\]
\[15\ :3 = 5\]
\[18\ :3 = 6\]
\[8\ :4 = 2\]
\[16\ :4 = 4\]
\[20\ :4 = 5\]
\[15\ :5 = 3\]
\[20\ :5 = 4\]
\[18\ :6 = 3\]