Calcul (nombres naturels)

$$\begin{array}{rrrrrr} & {\small\color{red}{6}} & {\small\color{red}{12}} & {\small\color{red}{12}} & {\small\color{red}{15}}\\ & \color{gray}{7} & \color{gray}{3} & \color{gray}{3} & \color{gray}{5}\\ - & & 5 & 7 & 9\\ \hline & 6 & 7 & 5 & 6\\ \end{array}$$

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