\[\boxed{\mathbf{27.}}\]
\[\textbf{а)}\ 5 + 4\ :2 = 5 + 2 = 7\]
\[15 - 6\ :2 = 15 - 3 = 12\]
\[8\ :2 + 25 = 4 + 25 = 29\]
\[10\ :2 + 31 = 5 + 31 = 36\]
\[26 - 12\ :2 = 26 - 6 = 20\]
\[16\ :2 + 21 = 8 + 21 = 29\]
\[\textbf{б)}\ 67 - 15\ :3 = 67 - 5 = 62\]
\[20\ :5 + 51 = 4 + 51 = 55\]
\[86 - 16\ :4 = 86 - 4 = 82\]