\[\boxed{\mathbf{20.}}\]
\[1)\ 7,\ 14,\ 21,\ 28,\ 35,\ 42,\ 49,\ 56,\ 63,\ \]
\[70.\]
\[2)\ 70,\ 63,\ 56,\ 49,\ 42,\ 35,\ 28,\ 21,\]
\[\ 14,\ 7.\]