\[\boxed{\mathbf{23}\mathbf{.}}\]
\[\textbf{а)}\ 3 \cdot 4 + 20 = 12 + 20 = 32\]
\[3 \cdot 6 + 5 = 18 + 5 = 23\]
\[30 + 2 \cdot 8 = 30 + 16 = 46\]
\[20 - 2 \cdot 6 = 20 - 12 = 8\]
\[30 - 3 \cdot 7 = 30 - 21 = 9\]
\[3 \cdot 10 - 12 = 30 - 12 = 18\]
\[\textbf{б)}\ 45 + 3 \cdot 9 = 45 + 27 = 72\]
\[71 - 3 \cdot 8 = 71 - 24 = 47\]
\[100 - 4 \cdot 4 = 100 - 16 = 84\]