\[\boxed{\mathbf{12.}}\]
\[\textbf{а)}\ 2 \cdot 8\ :4 = 16\ :4 = 4\]
\[14\ :7 \cdot 9 = 2 \cdot 9 = 18\]
\[5 \cdot 4\ :10 = 20\ :10 = 2\]
\[65 - (7 + 27) = 65 - 34 = 31\]
\[30 - 5 \cdot 5 = 30 - 25 = 5\]
\[100 - 6 \cdot 1 = 100 - 6 = 94\]
\[\textbf{б)}\ 72\ :8 \cdot 6 = 9 \cdot 6 = 54\]
\[8 \cdot 8 - 57 = 64 - 57 = 7\]
\[92 + 56\ :7 = 92 + 8 = 100\]