Решение:
1) \( \frac{5}{6} \cdot \frac{3}{20} = \frac{15}{120} = \frac{1}{8} \).
2) \( \frac{3}{7} \cdot \frac{6}{11} = \frac{18}{77} \).
3) \( \frac{10}{11} \cdot \frac{11}{26} = \frac{110}{286} = \frac{55}{143} \).
4) \( \frac{24}{39} \cdot \frac{65}{40} = \frac{1560}{1560} = 1 \).