\[\boxed{\mathbf{18.}}\]
\[\textbf{а)}\ 14\ :2 = 7\]
\[12\ :2 = 6\]
\[18\ :2 = 9\]
\[6\ :3 = 2\]
\[12\ :3 = 4\]
\[15\ :3 = 5\]
\[8\ :4 = 2\]
\[12\ :4 = 3\]
\[20\ :4 = 5\]
\[\textbf{б)}\ 12\ :3\ :2 = 4\ :2 = 2\]
\[18\ :3\ :2 = 6\ :2 = 3\]
\[16\ :4\ :2 = 4\ :2 = 2\]