\[\boxed{\mathbf{21.}}\]
\[\textbf{а)}\ 7 \cdot 4 = 28\]
\[7 \cdot 7 = 49\]
\[7 \cdot 8 = 56\]
\[5 \cdot 7 = 35\]
\[4 \cdot 7 = 28\]
\[6 \cdot 7 = 42\]
\[\textbf{б)}\ 7 \cdot 10 - 2 = 70 - 2 = 68\]
\[54 - 7 \cdot 6 = 54 - 42 = 12\]
\[68 - 7 \cdot 9 = 68 - 63 = 5\]
\[20 - 7 \cdot 2 = 20 - 14 = 6\]
\[30 - 7 \cdot 3 = 30 - 21 = 9\]
\[40 - 7 \cdot 5 = 40 - 35 = 5\]