Задание 1.

1. $$\frac{2}{7}\cdot\frac{42}{5} = \frac{2\cdot42}{7\cdot5} = \frac{2\cdot6\cdot7}{7\cdot5} = \frac{2\cdot6}{5} = \frac{12}{5} = 2,4$$
2. $$\frac{3}{16}:\frac{5}{8} = \frac{3}{16}\cdot\frac{8}{5} = \frac{3\cdot8}{16\cdot5} = \frac{3\cdot8}{2\cdot8\cdot5} = \frac{3}{2\cdot5} = \frac{3}{10} = 0,3$$
3. $$\frac{1}{10}-\frac{39}{50} = \frac{1\cdot5}{10\cdot5}-\frac{39}{50} = \frac{5}{50}-\frac{39}{50} = \frac{5-39}{50} = \frac{-34}{50} = \frac{-17}{25} = -0,68$$
4. $$\frac{1}{5}+\frac{3}{20} = \frac{1\cdot4}{5\cdot4}+\frac{3}{20} = \frac{4}{20}+\frac{3}{20} = \frac{4+3}{20} = \frac{7}{20} = 0,35$$