\[\boxed{\mathbf{25.}}\]
\[\textbf{а)}\ 5 + 2 \cdot 8 = 5 + 16 = 21\]
\[3 \cdot 9 - 12 = 27 - 12 = 15\]
\[3 + 6 \cdot 8 = 3 + 48 = 51\]
\[30 - 12\ :6 = 30 - 2 = 28\]
\[31 - 30\ :6 = 31 - 5 = 26\]
\[23 - 24\ :6 = 23 - 4 = 19\]
\[\textbf{б)}\ 42\ :6 + 57 = 7 + 57 = 64\]
\[46 + 6 \cdot 9 = 46 + 54 = 100\]
\[100 - 48\ :6 = 100 - 8 = 92\]