Ответ:

  1. \(\frac{5}{21}\cdot\frac{14}{15}=\frac{2}{9}\).
  2. \(\frac{7}{36}:\frac{21}{48}=\frac{7}{36}\cdot\frac{48}{21}=\frac{4}{9}\).
  3. \(3,05\cdot2,4=7,32\).
  4. \(28,56:0,8=35,7\).

Ответ: а) \(\frac{2}{9}\); б) \(\frac{4}{9}\); в) 7,32; г) 35,7.