\[\boxed{\mathbf{18.}}\]
\[\textbf{а)}\ 47 + 20 = 67\]
\[61 - 30 = 31\]
\[18 + 50 = 68\]
\[20 + 56 = 76\]
\[56 - 20 = 36\]
\[69 - 30 = 39\]
\[39 + 40 = 79\]
\[80 + 14 = 94\]
\[71 - 50 = 21\]
\[\textbf{б)}\ 50 + (6 + 6) = 50 + 12 = 62\]
\[80 + (7 + 7) = 80 + 14 = 94\]
\[(8 + 8) + 70 = 16 + 70 = 86\]