\[\boxed{\mathbf{5.}}\]
\[\textbf{а)}\ 47 + 20 = 67\]
\[61 + 30 = 91\]
\[18 + 50 = 68\]
\[20 + 56 = 76\]
\[40 + 24 = 64\]
\[60 + 18 = 78\]
\[39 + 40 = 79\]
\[70 + 14 = 84\]
\[61 + 30 = 91\]
\[\textbf{б)}\ 60 + (6 + 6) = 60 + 12 = 72\]
\[70 + (7 + 7) = 70 + 14 = 84\]
\[(8 + 8) + 80 = 16 + 80 = 96\]