\[\boxed{\mathbf{20.}}\]
\[2 \cdot 7 = 14\]
\[2 \cdot 8 = 16\]
\[2 \cdot 9 = 18\]
\[3 \cdot 3 = 9\]
\[3 \cdot 4 = 12\]
\[3 \cdot 5 = 15\]
\[4 \cdot 3 = 12\]
\[4 \cdot 4 = 16\]
\[4 \cdot 5 = 20\]
\[5 \cdot 2 = 10\]
\[5 \cdot 3 = 15\]
\[5 \cdot 4 = 20\]