\[\boxed{\mathbf{15.}}\]
\[1)\ 6,\ 12,\ 18,\ 24,\ 30,\ 36,\ 42,\ 48,\ 54,\ \]
\[60.\]
\[2)\ 60,\ 54,\ 48,\ 42,\ 36,\ 30,\ 24,\ 18,\ \]
\[12,\ 6.\]