\[\boxed{\mathbf{13.}}\]
\[\textbf{а)}\ 27\ :3 = 9\]
\[4 \cdot 6 = 24\]
\[32\ :4 = 8\]
\[24 - 7 \cdot 2 = 24 - 14 = 10\]
\[30\ :6 + 48 = 5 + 48 = 53\]
\[70 - 42\ :6 = 70 - 7 = 63\]
\[\textbf{б)}\ 64 - 9 \cdot 7 = 64 - 63 = 1\]
\[81\ :9 - 9 = 9 - 9 = 0\]
\[100 - 6 \cdot 0 = 100 - 0 = 100\]