\[\boxed{\mathbf{11.}}\]
\[\textbf{а)}\ 2 \cdot 3 = 6\]
\[3 \cdot 4 = 12\]
\[4 \cdot 2 = 8\]
\[15\ :3 = 5\]
\[20\ :2 = 10\]
\[18\ :6 = 3\]
\[\textbf{б)}\ 4 \cdot 4\ :2 = 16\ :2 = 8\]
\[12\ :2 \cdot 3 = 6 \cdot 3 = 18\]
\[16\ :4\ :2 = 4\ :2 = 2\]
\[2 \cdot 2 \cdot 2 = 4 \cdot 2 = 8\]
\[3 \cdot 2 \cdot 3 = 6 \cdot 3 = 18\]
\[20\ :2\ :2 = 10\ :2 = 5\]