\[\boxed{\mathbf{10}\mathbf{.}}\]
\[\textbf{а)}\ 2 \cdot 4 + 3 = 8 + 3 = 11\]
\[9 + 2 \cdot 2 = 9 + 4 = 13\]
\[20 - 2 \cdot 3 = 20 - 6 = 14\]
\[2 \cdot 5 + 21 = 10 + 21 = 31\]
\[\textbf{б)}\ 2 \cdot 9 + 82 = 18 + 82 = 100\]
\[100 - 2 \cdot 6 = 100 - 12 = 88\]