21. Найдите произведение: 1) 4/7 · 21; 2) 36 · 3/24; 3) 8/14 · 2/3; 4) 21/8 · 5/7; 5) 16/12 · 2/4.

1. \(\frac{4}{7}\cdot21=12\).
2. \(36\cdot\frac{3}{24}=\frac{9}{2}=4\frac{1}{2}\).
3. \(\frac{8}{14}\cdot\frac{2}{3}=\frac{8}{21}\).
4. \(\frac{21}{8}\cdot\frac{5}{7}=\frac{15}{8}=1\frac{7}{8}\).
5. \(\frac{16}{12}\cdot\frac{2}{4}=\frac{2}{3}\).

Ответ: 12; 4 1/2; 8/21; 1 7/8; 2/3.