\[\boxed{\mathbf{7.}}\]
\[\textbf{а)}\ 14 + 0 = 14\]
\[8 - 0 = 8\]
\[0 + 20 = 20\]
\[11 - 11 = 0\]
\[\textbf{б)}\ 2 \cdot 7 - 0 = 14 - 0 = 14\]
\[0 + 3 \cdot 4 = 0 + 12 = 12\]
\[18\ :3 - 0 = 6 - 0 = 6\]
\[20\ :5 - 4 = 4 - 4 = 0\]