\[\boxed{\mathbf{27}\mathbf{.}}\]
\[\textbf{а)}\ 3 \cdot 8 - 5 = 24 - 5 = 19\]
\[4 \cdot 7 + 5 = 28 + 5 = 33\]
\[30 + 24\ :4 = 30 + 6 = 36\]
\[30 - 27\ :3 = 30 - 9 = 21\]
\[\textbf{б)}\ 100 - 4 \cdot 9 = 100 - 36 = 64\]
\[71 - 28\ :4 = 71 - 7 = 64\]