Basis Optellen

Reken uit

\(3 + 9 = \)

\(\begin{align} & 3 + 9 \\= & 9 + 3 \\= & 9 + \color{green}{1} + \color{red}{2} \\= & \color{green}{10} + \color{red}{2} \\= & \color{red}{12} \end{align}\)