\[\boxed{\mathbf{28.}}\]
\[\textbf{а)}\ 2 \cdot 8 = 16\]
\[10\ :2 = 5\]
\[2 \cdot 9 = 18\]
\[12\ :2 = 6\]
\[2 \cdot 7 = 14\]
\[16\ :2 = 8\]
\[\textbf{б)}\ 2 \cdot 10\ :4 = 20\ :4 = 5\]
\[9\ :3 \cdot 5 = 3 \cdot 5 = 15\]
\[4 \cdot 4\ :2 = 16\ :2 = 8\]
\[8\ :2 \cdot 5 = 4 \cdot 5 = 20\]
\[6 \cdot 2\ :4 = 12\ :4 = 3\]
\[18\ :2\ :3 = 9\ :3 = 3\]