28. Выполни действия: 1) 4/21 · 7/16 = 8/9; 2) 3 1/9 · 5/2 = 5 2/3; 3) 4 1/5 · 2 1/7 = 22 1/2; 4) 1 7/8 · 1 1/9 · 23/25 = 18; 5) 8/21 · 1,05 · 1 2/3 = 4 11/20.

1. \(\frac{4}{21}\cdot\frac{7}{16}=\frac{1}{12}\).
2. \(3\frac{1}{9}\cdot\frac{5}{2}=\frac{28}{9}\cdot\frac{5}{2}=\frac{70}{9}=7\frac{7}{9}\).
3. \(4\frac{1}{5}\cdot2\frac{1}{7}=\frac{21}{5}\cdot\frac{15}{7}=9\).
4. \(1\frac{7}{8}\cdot1\frac{1}{9}\cdot\frac{23}{25}=\frac{15}{8}\cdot\frac{10}{9}\cdot\frac{23}{25}=\frac{23}{12}=1\frac{11}{12}\).
5. \(\frac{8}{21}\cdot1.05\cdot1\frac{2}{3}=\frac{8}{21}\cdot\frac{21}{20}\cdot\frac{5}{3}=\frac{2}{3}\).

Ответ: 1) \(\frac{1}{12}\); 2) \(7\frac{7}{9}\); 3) \(9\); 4) \(1\frac{11}{12}\); 5) \(\frac{2}{3}\).