78 * 85 − 69 = 6630 − 69 = 6561
1)
$\begin{array}{r}\times \underset{¯}{\begin{array}{r}78\\ 85\end{array}}\\ 390\\ 624\phantom{0}\\ \overline{6630}\end{array}$
2)
$\begin{array}{r}\begin{array}{r}-\end{array}\underline{\begin{array}{r}6630\\ 69\end{array}}\\ \begin{array}{r}\mathrm{6561 }\end{array}\end{array}$

78 * (85 − 69) = 78 * 16 = 1248
1)
$\begin{array}{r}\begin{array}{r}-\end{array}\underline{\begin{array}{r}85\\ 69\end{array}}\\ \begin{array}{r}\mathrm{16 }\end{array}\end{array}$
2)
$\begin{array}{r}\times \underset{¯}{\begin{array}{r}78\\ 16\end{array}}\\ 468\\ 78\phantom{0}\\ \overline{1248}\end{array}$