Ответ:

  1. а) \(\frac{1}{5}+\frac{3}{20}=\frac{4}{20}+\frac{3}{20}=\frac{7}{20}\).
  2. б) \(\frac{1}{10}-\frac{39}{50}=\frac{5}{50}-\frac{39}{50}=-\frac{34}{50}=-\frac{17}{25}\).
  3. в) \(\frac{2}{7}\cdot\frac{42}{5}=\frac{2\cdot6}{5}=\frac{12}{5}\).
  4. г) \(\frac{3}{16}:\frac{5}{8}=\frac{3}{16}\cdot\frac{8}{5}=\frac{3}{10}\).