- \(\frac{2}{7}\cdot\frac{42}{5}=\frac{2\cdot6}{5}=\frac{12}{5}=2\frac{2}{5}\).
- \(\frac{3}{16}\cdot\frac{5}{8}=\frac{15}{128}\).
- \(\frac{1}{10}-\frac{39}{50}=\frac{5}{50}-\frac{39}{50}=-\frac{34}{50}=-\frac{17}{25}\).
- \(\frac{1}{5}+\frac{3}{20}=\frac{4}{20}+\frac{3}{20}=\frac{7}{20}\).
Ответ: 1) \(2\frac{2}{5}\); 2) \(\frac{15}{128}\); 3) \(-\frac{17}{25}\); 4) \(\frac{7}{20}\).