80. Найдите значение выражения: 1) 5 1/3 : 8/9 : 1/3; 2) 5 1/3 · (8/9 : 1/3); 3) (8 5/7 − 6 5/6 : 1 5/36) : 4 3/4; 4) (2 5/6 + 2 2/9) : 3 1/4 − 2/7 : 1 2/7.

1. \(5\frac{1}{3}:\frac{8}{9}:\frac{1}{3}=\frac{16}{3}\cdot\frac{9}{8}\cdot3=18\).
2. \(5\frac{1}{3}\cdot(\frac{8}{9}:\frac{1}{3})=\frac{16}{3}\cdot\frac{8}{3}=14\frac{2}{9}\).
3. \((8\frac{5}{7}-6\frac{5}{6}:1\frac{5}{36}):4\frac{3}{4}=(\frac{61}{7}-6):\frac{19}{4}=\frac{19}{7}\cdot\frac{4}{19}=\frac{4}{7}\).
4. \((2\frac{5}{6}+2\frac{2}{9}):3\frac{1}{4}-\frac{2}{7}:1\frac{2}{7}=\frac{14}{9}-\frac{2}{9}=\frac{4}{3}\).

Ответ: 1) \(18\); 2) \(14\frac{2}{9}\); 3) \(\frac{4}{7}\); 4) \(\frac{4}{3}\).