\[\boxed{\mathbf{15.}}\]
\[\textbf{а)}\ 20 + 2 \cdot 4 = 20 + 8 = 28\]
\[20 - 2 \cdot 3 = 20 - 6 = 14\]
\[30 + 2 \cdot 8 = 30 + 16 = 46\]
\[30 - 2 \cdot 6 = 30 - 12 = 18\]
\[\textbf{б)}\ 46 + 2 \cdot 7 = 46 + 14 = 60\]
\[46 - 2 \cdot 8 = 46 - 16 = 30\]
\[88 + 3 \cdot 4 = 88 + 12 = 100\]
\[88 - 4 \cdot 3 = 88 - 12 = 76\]