\[\boxed{\mathbf{8}\mathbf{.}}\]
\[\textbf{а)}\ 28 + 2 = 30\]
\[34 + 13 = 47\]
\[45 - 3 = 42\]
\[40 - 12 = 28\]
\[\textbf{б)}\ 63 - (4 + 8) = 63 - 12 = 51\]
\[74 + (13 - 7) = 74 + 6 = 80\]
\[80 - 25 = 55\]
\[100 - 42 = 58\]