Ответ:

  1. \(\frac{9}{10}-\frac{1}{5}=\frac{9}{10}-\frac{2}{10}=\frac{7}{10}\), поэтому \(\frac{6}{5}:\frac{7}{10}=\frac{6}{5}\cdot\frac{10}{7}=\frac{12}{7}\).
  2. \(\frac{7}{25}+\frac{1}{5}=\frac{7}{25}+\frac{5}{25}=\frac{12}{25}\), поэтому \(\frac{12}{25}:\frac{3}{50}=\frac{12}{25}\cdot\frac{50}{3}=8\).
  3. \(\frac{3}{7}:\frac{15}{28}=\frac{3}{7}\cdot\frac{28}{15}=\frac{4}{5}\), поэтому \(1-\frac{4}{5}=\frac{1}{5}\).
  4. \(\frac{34}{9}:\frac{17}{54}\cdot\frac{5}{6}=\frac{34}{9}\cdot\frac{54}{17}\cdot\frac{5}{6}=20\).

Ответ: а) \(\frac{12}{7}\); б) \(8\); в) \(\frac{1}{5}\); г) \(20\).