\[\boxed{\mathbf{8.}}\]
\[\textbf{а)}\ 42\ :6 = 7\]
\[42\ :7 = 6\]
\[49\ :7 = 7\]
\[70\ :7 = 10\]
\[56\ :7 = 8\]
\[63\ :7 = 9\]
\[\textbf{б)}\ 60 - 28\ :7 = 60 - 4 = 56\]
\[100 - 35\ :7 = 100 - 5 = 95\]