Ответ:

  1. \(4\cdot\frac{5}{21}=\frac{20}{21}\).
  2. Сокращаем \(24\) и \(12\): \(24\cdot\frac{7}{12}=2\cdot7=14\).
  3. Сокращаем \(9\) и \(63\): \(\frac{16}{63}\cdot9=\frac{16}{7}=2\frac{2}{7}\).
  4. Сокращаем \(36\) и \(48\): \(36\cdot\frac{13}{48}=\frac{3}{4}\cdot13=\frac{39}{4}=9\frac{3}{4}\).

Ответ: 1) \(\frac{20}{21}\); 2) 14; 3) \(2\frac{2}{7}\); 4) \(9\frac{3}{4}\).