\[\boxed{\mathbf{31.}}\]
\[6 \cdot 6 < 6 \cdot 7\]
\[36 < 42\]
\[6 \cdot 9 > 6 \cdot 8\]
\[54 > 48\]
\[3 \cdot 6 = 2 \cdot 9\]
\[18 = 18\]
\[4 \cdot 6 = 6 \cdot 4\]
\[24 = 24\]
\[5 \cdot 6 = 6 \cdot 5\]
\[30 = 30\]
\[3 \cdot 9 < 4 \cdot 7\]
\[27 < 28\]