%@P:exocorcp
%@Auteur: d'après IREM Strasbourg
\begin{center}
  \begin{tabular}{|>{\columncolor[rgb]{1,1,0}}*{8}{c|}}
\hline
\rowcolor{yellow}$\times$&&&&\phantom{15}&8&&\\
\hline
&&8&&&&&20\\
\hline
&&&&&16&&\\
\hline
\phantom{15}&&&15&&&20&\\
\hline
&&&9&&&&\\
\hline
7&49&&&7&&&\\
\hline
&&18&&&72&&\\
\hline
&&&&&&24&\\
\hline
  \end{tabular}
\end{center}
%@Correction:
\begin{center}
  \begin{tabular}{|>{\columncolor[rgb]{1,1,0}}*{8}{c|}}
\hline
\rowcolor{yellow}$\times$&\opcopy{7}{a1}\textcolor{red}{\opprint{a1}}&\opcopy{2}{a2}\textcolor{red}{\opprint{a2}}&\opcopy{3}{a3}\textcolor{red}{\opprint{a3}}&\opcopy{1}{a4}\textcolor{red}{\opprint{a4}}&\opcopy{8}{a5}\opprint{a5}&\opcopy{4}{a6}\textcolor{red}{\opprint{a6}}&\opcopy{5}{a7}\textcolor{red}{\opprint{a7}}\\
\hline
\opcopy{4}{b1}\textcolor{red}{\opprint{b1}}&\opmul*{b1}{a1}{a}\textcolor{red}{\opprint{a}}&\opmul*{b1}{a2}{a}\opprint{a}&\opmul*{b1}{a3}{a}\textcolor{red}{\opprint{a}}&\opmul*{b1}{a4}{a}\textcolor{red}{\opprint{a}}&\opmul*{b1}{a5}{a}\textcolor{red}{\opprint{a}}&\opmul*{b1}{a6}{a}\textcolor{red}{\opprint{a}}&\opmul*{b1}{a7}{a}\opprint{a}\\
\hline
\opcopy{2}{b2}\textcolor{red}{\opprint{b2}}&\opmul*{b2}{a1}{a}\textcolor{red}{\opprint{a}}&\opmul*{b2}{a2}{a}\textcolor{red}{\opprint{a}}&\opmul*{b2}{a3}{a}\textcolor{red}{\opprint{a}}&\opmul*{b2}{a4}{a}\textcolor{red}{\opprint{a}}&\opmul*{b2}{a5}{a}\opprint{a}&\opmul*{b2}{a6}{a}\textcolor{red}{\opprint{a}}&\opmul*{b2}{a7}{a}\textcolor{red}{\opprint{a}}\\
\hline
\opcopy{5}{b3}\textcolor{red}{\opprint{b3}}&\opmul*{b3}{a1}{a}\textcolor{red}{\opprint{a}}&\opmul*{b3}{a2}{a}\textcolor{red}{\opprint{a}}&\opmul*{b3}{a3}{a}\opprint{a}&\opmul*{b3}{a4}{a}\textcolor{red}{\opprint{a}}&\opmul*{b3}{a5}{a}\textcolor{red}{\opprint{a}}&\opmul*{b3}{a6}{a}\opprint{a}&\opmul*{b3}{a7}{a}\textcolor{red}{\opprint{a}}\\
\hline
\opcopy{3}{b4}\textcolor{red}{\opprint{b4}}&\opmul*{b4}{a1}{a}\textcolor{red}{\opprint{a}}&\opmul*{b4}{a2}{a}\textcolor{red}{\opprint{a}}&\opmul*{b4}{a3}{a}\opprint{a}&\opmul*{b4}{a4}{a}\textcolor{red}{\opprint{a}}&\opmul*{b4}{a5}{a}\textcolor{red}{\opprint{a}}&\opmul*{b4}{a6}{a}\textcolor{red}{\opprint{a}}&\opmul*{b4}{a7}{a}\textcolor{red}{\opprint{a}}\\
\hline
\opcopy{7}{b5}\opprint{b5}&\opmul*{b5}{a1}{a}\opprint{a}&\opmul*{b5}{a2}{a}\textcolor{red}{\opprint{a}}&\opmul*{b5}{a3}{a}\textcolor{red}{\opprint{a}}&\opmul*{b5}{a4}{a}\opprint{a}&\opmul*{b5}{a5}{a}\textcolor{red}{\opprint{a}}&\opmul*{b5}{a6}{a}\textcolor{red}{\opprint{a}}&\opmul*{b5}{a7}{a}\textcolor{red}{\opprint{a}}\\
\hline
\opcopy{9}{b6}\textcolor{red}{\opprint{b6}}&\opmul*{b6}{a1}{a}\textcolor{red}{\opprint{a}}&\opmul*{b6}{a2}{a}\opprint{a}&\opmul*{b6}{a3}{a}\textcolor{red}{\opprint{a}}&\opmul*{b6}{a4}{a}\textcolor{red}{\opprint{a}}&\opmul*{b6}{a5}{a}\opprint{a}&\opmul*{b6}{a6}{a}\textcolor{red}{\opprint{a}}&\opmul*{b6}{a7}{a}\textcolor{red}{\opprint{a}}\\
\hline
\opcopy{6}{b7}\textcolor{red}{\opprint{b7}}&\opmul*{b7}{a1}{a}\textcolor{red}{\opprint{a}}&\opmul*{b7}{a2}{a}\textcolor{red}{\opprint{a}}&\opmul*{b7}{a3}{a}\textcolor{red}{\opprint{a}}&\opmul*{b7}{a4}{a}\textcolor{red}{\opprint{a}}&\opmul*{b7}{a5}{a}\textcolor{red}{\opprint{a}}&\opmul*{b7}{a6}{a}\opprint{a}&\opmul*{b7}{a7}{a}\textcolor{red}{\opprint{a}}\\
\hline
  \end{tabular}
\end{center}