\[\boxed{\mathbf{11.}}\]
\[\textbf{а)}\ 2 \cdot 5 = 10\]
\[8\ :2 = 4\]
\[2 \cdot 6 = 12\]
\[10\ :2 = 5\]
\[2 \cdot 8 = 16\]
\[14\ :2 = 7\]
\[\textbf{б)}\ 2 \cdot 9\ :3 = 18\ :3 = 6\]
\[6\ :3 \cdot 7 = 2 \cdot 7 = 14\]
\[4 \cdot 5\ :2 = 20\ :2 = 10\]
\[4\ :2 \cdot 10 = 2 \cdot 10 = 20\]
\[3 \cdot 6\ :2 = 18\ :2 = 9\]
\[16\ :4 \cdot 5 = 4 \cdot 5 = 20\]