\[\boxed{\mathbf{12.}}\]
\[3 \cdot 4 = 12\]
\[4 \cdot 3 = 12\]
\[12\ :3 = 4\]
\[12\ :4 = 3\]
\[4 \cdot 5 = 20\]
\[5 \cdot 4 = 20\]
\[20\ :4 = 5\]
\[20\ :5 = 4\]
\[6 \cdot 3 = 18\]
\[3 \cdot 6 = 18\]
\[18\ :3 = 6\]
\[18\ :6 = 3\]
\[3 \cdot 5 = 15\]
\[5 \cdot 3 = 15\]
\[15\ :3 = 5\]
\[15\ :5 = 3\]