б) \(\frac{6}{25} \cdot \frac{20}{21}\) = \(\frac{6 \cdot 20}{25 \cdot 21}\) = \(\frac{2 \cdot 3 \cdot 4 \cdot 5}{5 \cdot 5 \cdot 3 \cdot 7}\) = \(\frac{2 \cdot 4}{5 \cdot 7}\) = \(\frac{8}{35}\)
Ответ: \(\frac{8}{35}\)