\[\boxed{\mathbf{8.}}\]
\[\textbf{а)}\ 20 - (11 + 4) = 20 - 15 = 5\]
\[12 - (1 + 10) = 12 - 11 = 1\]
\[\textbf{б)}\ 40 + (25 - 20) = 40 + 5 = 45\]
\[70 + (35 - 30) = 70 + 5 = 75\]
\[50 - (15 - 5) = 50 - 10 = 40\]
\[90 - (18 - 8) = 90 - 10 = 80\]