3) $$(19 \frac{5}{21} - 8 \frac{3}{14}) - 6 \frac{5}{7};$$ 4) $$(19 - 8 \frac{7}{8}) - (4 \frac{5}{6} - 2 \frac{8}{9}).$$

Решим примеры.

1. 3)

   $$(19 \frac{5}{21} - 8 \frac{3}{14}) - 6 \frac{5}{7} = (19 \frac{10}{42} - 8 \frac{9}{42}) - 6 \frac{5}{7} = 11 \frac{1}{42} - 6 \frac{5}{7} = 11 \frac{1}{42} - 6 \frac{30}{42} = 10 \frac{43}{42} - 6 \frac{30}{42} = 4 \frac{13}{42}.$$

   Ответ: $$4 \frac{13}{42}$$

2. 4)

   $$(19 - 8 \frac{7}{8}) - (4 \frac{5}{6} - 2 \frac{8}{9}) = (18 \frac{8}{8} - 8 \frac{7}{8}) - (4 \frac{15}{18} - 2 \frac{16}{18}) = 10 \frac{1}{8} - (3 \frac{33}{18} - 2 \frac{16}{18}) = 10 \frac{1}{8} - 1 \frac{17}{18} = 10 \frac{9}{72} - 1 \frac{68}{72} = 9 \frac{81}{72} - 1 \frac{68}{72} = 8 \frac{13}{72}.$$

   Ответ: $$8 \frac{13}{72}$$