\[\boxed{\mathbf{42.}}\]
\[\textbf{а)}\ 4 \cdot 6 - 6 = 24 - 6 = 18\]
\[28 + 24\ :3 = 28 + 8 = 36\]
\[18\ :6 \cdot 7 = 3 \cdot 7 = 21\]
\[34 + 16 - 7 = 50 - 7 = 43\]
\[80 - 2 \cdot 8 = 80 - 16 = 64\]
\[32\ :4 + 47 = 8 + 47 = 55\]
\[\textbf{б)}\ 29 + 43 - 65 = 72 - 65 = 7\]
\[55 - 8 \cdot 6 = 55 - 48 = 7\]
\[63\ :7 + 84 = 9 + 84 = 93\]