Решение:

1. $$\frac{21}{40}:\frac{3}{4} = \frac{21}{40}*\frac{4}{3} = \frac{21*4}{40*3} = \frac{7*1}{10*1} = \frac{7}{10}$$
2. $$1\frac{5}{9}:1\frac{8}{9} = \frac{1*9+5}{9} : \frac{1*9+8}{9} = \frac{14}{9} : \frac{17}{9} = \frac{14}{9} * \frac{9}{17} = \frac{14*9}{9*17} = \frac{14}{17}$$
3. $$5:\frac{15}{16} = 5 * \frac{16}{15} = \frac{5*16}{15} = \frac{1*16}{3} = \frac{16}{3} = 5\frac{1}{3}$$
4. $$\frac{9}{17}:3 = \frac{9}{17} * \frac{1}{3} = \frac{9*1}{17*3} = \frac{3*1}{17*1} = \frac{3}{17}$$

1. $$\frac{7}{10}$$
2. $$\frac{14}{17}$$
3. $$5\frac{1}{3}$$
4. $$\frac{3}{17}$$