\[\boxed{\mathbf{24.}}\]
\[\textbf{а)}\ 50 + 2 \cdot 9 = 50 + 18 = 68\]
\[50 - 2 \cdot 4 = 50 - 8 = 42\]
\[\textbf{б)}\ 61 + 3 \cdot 3 = 61 + 9 = 70\]
\[78 - 3 \cdot 6 = 78 - 18 = 60\]
\[84 + 4 \cdot 4 = 84 + 16 = 100\]
\[100 - 5 \cdot 3 = 100 - 15 = 85\]