\[\boxed{\mathbf{10.}}\]
\[\textbf{а)}\ 32 - 2 \cdot 7 = 32 - 14 = 18\]
\[16 + 16\ :4 = 16 + 4 = 20\]
\[4 \cdot 6 - 9 = 24 - 9 = 15\]
\[33 - 21\ :3 = 33 - 7 = 26\]
\[33 - 21 - 3 = 12 - 3 = 9\]
\[33 - (21 - 3) = 33 - 18 = 15\]
\[\textbf{б)}\ 81 - 8 \cdot 7 = 81 - 56 = 25\]
\[63 - 42\ :7 = 63 - 6 = 57\]
\[54\ :6 - 9 = 9 - 9 = 0\]