\[\boxed{\mathbf{23.}}\]
\[\textbf{а)}\ 74 - 6 + 20 = 68 + 20 = 88\]
\[4 + 28 - 3 = 32 - 3 = 29\]
\[80 - (8 + 8) = 80 - 16 = 64\]
\[90 - 6 \cdot 6 = 90 - 36 = 54\]
\[\textbf{б)}\ 67 - 29 + 42 = 38 + 42 = 80\]
\[72 - 72\ :8 = 72 - 9 = 63\]