\[\boxed{\mathbf{20.}}\]
\[6 \cdot 2 - 6 = 12 - 6 = 6\]
\[6 \cdot 3 - 6 = 18 - 6 = 12\]
\[6 \cdot 4 - 6 = 24 - 6 = 18\]
\[6 \cdot 5 - 6 = 30 - 6 = 24\]
\[6 \cdot 6 - 6 = 36 - 6 = 30\]
\[6 \cdot 7 - 6 = 42 - 6 = 36\]
\[6 \cdot 8 - 6 = 48 - 6 = 42\]
\[6 \cdot 9 - 6 = 54 - 6 = 48\]