\[\boxed{\mathbf{795\ (795)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\textbf{а)} - 21 = 7 \bullet ( - 3)\]
\[- 21 = - 7 \bullet 3\]
\[- 21 = - 21 \bullet 1\]
\[- 21 = 21 \bullet ( - 1).\]
\[\textbf{б)}\ 20 = 4 \bullet 5\]
\[20 = ( - 4) \bullet ( - 5)\]
\[20 = 10 \bullet 2\]
\[20 = ( - 10) \bullet ( - 2)\]
\[20 = 20 \bullet 1\]
\[20 = - 20 \bullet ( - 1)\]
\[\textbf{в)} - 1 = - 1 \bullet 1\]
\[\textbf{г)}\ 1 = 1 \bullet 1\]
\[1 = ( - 1) \bullet ( - 1)\]
\[\textbf{д)}\ 0 \Longrightarrow бесконечно\ много\ вариантов:\]
\[0 = 0 \bullet k;\ \ где\ k - любое\ целое\ число.\]
\[Ответ:а)\ 4;\ \ б)\ 6;\ \ в)\ 1;\ \ г)\ 2;\ \ д)\ бесконечно\ много.\]