1. Выполните действия: a) $$\frac{7}{8} \times \frac{16}{21}$$; б) $$1\frac{8}{25} - 1\frac{4}{11}$$; в) $$1\frac{5}{7} \div 1\frac{17}{18}$$; г) $$5 \cdot 3\frac{7}{15}$$

a) $$\frac{7}{8} \times \frac{16}{21} = \frac{7 \times 16}{8 \times 21} = \frac{7 \times 2 \times 8}{8 \times 3 \times 7} = \frac{2}{3}$$

б) $$1\frac{8}{25} - 1\frac{4}{11} = \frac{33}{25} - \frac{15}{11} = \frac{33 \times 11}{25 \times 11} - \frac{15 \times 25}{11 \times 25} = \frac{363}{275} - \frac{375}{275} = -\frac{12}{275}$$

в) $$1\frac{5}{7} \div 1\frac{17}{18} = \frac{12}{7} \div \frac{35}{18} = \frac{12}{7} \times \frac{18}{35} = \frac{12 \times 18}{7 \times 35} = \frac{216}{245}$$

г) $$5 \cdot 3\frac{7}{15} = 5 \cdot \frac{52}{15} = \frac{5 \times 52}{15} = \frac{5 \times 52}{5 \times 3} = \frac{52}{3} = 17\frac{1}{3}$$

Ответ: a) $$\frac{2}{3}$$; б) $$-\frac{12}{275}$$; в) $$\frac{216}{245}$$; г) $$17\frac{1}{3}$$