20. \(5\frac{4}{7}\cdot1\frac{2}{5}=\)

Преобразуем смешанные числа:

\(5\frac{4}{7}=\frac{39}{7}\), \(1\frac{2}{5}=\frac{7}{5}\).

\(\frac{39}{7}\cdot\frac{7}{5}=\frac{39}{5}=7\frac{4}{5}\).

Ответ: \(7\frac{4}{5}\)