a) 7/8 * 16/21 ; 6) 1 8/25 * 1 4/11 ; в) 1 5/7 * 17/18 : г) 5.3 * 7/15

а) $$ \frac{7}{8} \cdot \frac{16}{21} = \frac{7 \cdot 16}{8 \cdot 21} = \frac{7 \cdot 8 \cdot 2}{8 \cdot 7 \cdot 3} = \frac{2}{3}$$

б) $$1 \frac{8}{25} \cdot 1 \frac{4}{11} = \frac{33}{25} \cdot \frac{15}{11} = \frac{3 \cdot 11 \cdot 3 \cdot 5}{5 \cdot 5 \cdot 11} = \frac{9}{5} = 1 \frac{4}{5}$$

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

г) $$5.3 \cdot \frac{7}{15} = \frac{53}{10} \cdot \frac{7}{15} = \frac{53 \cdot 7}{10 \cdot 15} = \frac{371}{150} = 2 \frac{71}{150}$$

Ответ: а) 2/3; б) 1 4/5; в) 216/245; г) 2 71/150