25a * 4 = (25 * 4) * a = 100a
Если a = 39, то 100a = 100 * 39 = 3900

5m * 20n = (5 * 20) * (m * n) = 100mn
Если m = 12, n = 7, то 100 * 12 * 7 = 100 * (12 * 7) = 100 * 84 = 8400

Вычисления
$\begin{array}{r}\times \underset{¯}{\begin{array}{r}12\\ 7\end{array}}\\ 84\end{array}$

125c * 8d = (125 * 8) * (c * d) = 1000cd
Если c = 16, d = 34, то 1000 * 16 * 34 = 1000 * (16 * 34) = 1000 * 544 = 544000

Вычисления
$\begin{array}{r}\times \underset{¯}{\begin{array}{r}16\\ 34\end{array}}\\ 64\\ 48\phantom{0}\\ \overline{544}\end{array}$