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