Решение:

1. a) $$-7 \cdot \frac{5}{21} = - \frac{7 \cdot 5}{21} = - \frac{35}{21} = - \frac{5}{3} = -1\frac{2}{3}$$
2. б) $$2\frac{4}{5} : 1\frac{13}{15} = \frac{14}{5} : \frac{28}{15} = \frac{14}{5} \cdot \frac{15}{28} = \frac{14 \cdot 15}{5 \cdot 28} = \frac{210}{140} = \frac{3}{2} = 1 \frac{1}{2}$$
3. в) $$- \frac{1}{5} + \frac{2}{3} = - \frac{3}{15} + \frac{10}{15} = \frac{7}{15}$$

Ответ: a) $$-1\frac{2}{3}$$; б) $$1\frac{1}{2}$$; в) $$\frac{7}{15}$$