б) \(\frac{3}{4} \cdot \frac{10}{13} \cdot \frac{39}{40}\) = \(\frac{3 \cdot 10 \cdot 39}{4 \cdot 13 \cdot 40}\) = \(\frac{1170}{2080}\). Сокращаем на 10: \(\frac{117}{208}\). Сокращаем на 13: \(\frac{117:13}{208:13}\) = \(\frac{9}{16}\). Ответ: \(\frac{9}{16}\)