\[\boxed{\mathbf{24.}}\]
\[\textbf{а)}\ 40 + 6\ :2 = 40 + 3 = 43\]
\[40 - 8\ :2 = 40 - 4 = 36\]
\[50 + 10\ :2 = 50 + 5 = 55\]
\[50 - 12\ :2 = 50 - 6 = 44\]
\[\textbf{б)}\ 76 + 12\ :3 = 76 + 4 = 80\]
\[76 - 12\ :4 = 76 - 3 = 73\]
\[97 + 15\ :5 = 97 + 3 = 100\]
\[97 - 15\ :3 = 97 - 5 = 92\]