\[\boxed{\mathbf{24.}}\]
\[\textbf{а)}\ 15 + 0 = 15\]
\[42 + 0 = 42\]
\[20 - 0 = 20\]
\[45 - 0 = 45\]
\[\textbf{б)}\ 29 + 1 + 0 = 30 + 0 = 30\]
\[50 - 0 - 1 = 50 - 1 = 49\]
\[73 + 10 + 0 = 83 + 0 = 83\]
\[95 - 10 - 0 = 85 - 0 = 85\]