\( \frac{3}{8} \cdot \frac{16}{21} = \frac{3 \cdot 16}{8 \cdot 21} = \frac{3 \cdot (2 \cdot 8)}{(8) \cdot (3 \cdot 7)} = \frac{2}{7} \)
Ответ: \(\frac{2}{7}\)