\[\boxed{\mathbf{484.}}\]
\[42\ :2 = 21\]
\[1)\ 42 = 40 + 2\]
\[2)\ 40\ :2 = 20\]
\[3)\ 2\ :2 = 1\]
\[4)\ 20 + 1 = 21\]
\[Проверка:\]
\[21 \cdot 2 = 42.\]