\[\boxed{\mathbf{8}\mathbf{.}}\]
\[\textbf{б)}\ 50 - 2 \cdot 9 = 50 - 18 = 32\]
\[40 + 20\ :4 = 40 + 5 = 45\]
\[3 \cdot 8 + 36 = 24 + 36 = 60\]
\[64 + 4 \cdot 9 = 64 + 36 = 100\]
\[100 - 32\ :4 = 100 - 8 = 92\]
\[21\ :3 + 0 = 7 + 0 = 7\]