\[\boxed{\mathbf{25.}}\]
\[\textbf{а)}\ 9 + 5 = 14\]
\[8 + 3 = 11\]
\[11 - 3 = 8\]
\[12 - 5 = 7\]
\[\textbf{б)}\ 12 - 4 + 7 = 8 + 7 = 15\]
\[12 - (4 + 7) = 12 - 11 = 1\]
\[9 + (20 - 12) = 9 + 8 = 17\]
\[(6 + 7) - 12 = 13 - 12 = 1\]