\[\boxed{\mathbf{30.}}\]
\[\textbf{а)}\ 25 + 10\ :2 = 25 + 5 = 30\]
\[36 - 12\ :2 = 36 - 6 = 30\]
\[90 - 16\ :4 = 90 - 4 = 86\]
\[32 + 16\ :2 = 32 + 8 = 40\]
\[70 - 14\ :2 = 70 - 7 = 63\]
\[77 + 15\ :5 = 77 + 3 = 80\]
\[\textbf{б)}\ 84 + 18\ :3 = 84 + 6 = 90\]
\[100 - 20\ :4 = 100 - 5 = 95\]
\[18\ :2 + 61 = 9 + 61 = 70\]