Решим пример по действиям:
- \(\frac{11}{15} \cdot \frac{21}{22} = \frac{11 \cdot 21}{15 \cdot 22} = \frac{1 \cdot 7}{5 \cdot 2} = \frac{7}{10}\)
- \(\frac{7}{10} \cdot \frac{9}{28} = \frac{7 \cdot 9}{10 \cdot 28} = \frac{1 \cdot 9}{10 \cdot 4} = \frac{9}{40}\)
- \(\frac{9}{40} \cdot \frac{8}{9} = \frac{9 8}{40 9} = \frac{1 1}{5 1} = \frac{1}{5}\)
Ответ: \(\frac{1}{5}\).