Машина прибавила число 5:
20 - 15 = 5
$\begin{array}{r}\begin{array}{r}-\end{array}\underline{\begin{array}{r}20\\ 15\end{array}}\\ \begin{array}{r}5\end{array}\end{array}$
15 + 5 = 20
$\begin{array}{r}\begin{array}{r}+\end{array}\underline{\begin{array}{r}15\\ 5\end{array}}\\ \begin{array}{r}20\end{array}\end{array}$
Машина вычла число 10:
40 - 30 = 10
$\begin{array}{r}\begin{array}{r}-\end{array}\underline{\begin{array}{r}40\\ 30\end{array}}\\ \begin{array}{r}10\end{array}\end{array}$
40 - 10 = 30
$\begin{array}{r}\begin{array}{r}-\end{array}\underline{\begin{array}{r}40\\ 10\end{array}}\\ \begin{array}{r}30\end{array}\end{array}$