\[\boxed{\mathbf{3.}}\]
\[2 \cdot 6 = 12\]
\[2 \cdot 8 = 16\]
\[2 \cdot 9 = 18\]
\[3 \cdot 5 = 15\]
\[3 \cdot 6 = 18\]
\[3 \cdot 4 = 12\]
\[4 \cdot 3 = 12\]
\[4 \cdot 4 = 16\]
\[4 \cdot 5 = 20\]