1. Вычислите: 1) 21/40 : 3/4; 2) 1 5/9 : 1 8/27; 3) 5 : 15/16; 4) 4 9/17 : 3.

1. \(\dfrac{21}{40}:\dfrac{3}{4}=\dfrac{21}{40}\cdot\dfrac{4}{3}=\dfrac{7}{10}\).
2. \(1\dfrac{5}{9}=\dfrac{14}{9}\), \(1\dfrac{8}{27}=\dfrac{35}{27}\).
   \(\dfrac{14}{9}:\dfrac{35}{27}=\dfrac{14}{9}\cdot\dfrac{27}{35}=\dfrac{6}{5}=1\dfrac{1}{5}\).
3. \(5:\dfrac{15}{16}=5\cdot\dfrac{16}{15}=\dfrac{16}{3}=5\dfrac{1}{3}\).
4. \(4\dfrac{9}{17}=\dfrac{77}{17}\).
   \(\dfrac{77}{17}:3=\dfrac{77}{17}\cdot\dfrac{1}{3}=\dfrac{77}{51}=1\dfrac{26}{51}\).

Ответ: 1) 7/10; 2) 1 1/5; 3) 5 1/3; 4) 1 26/51.