640. Найдите значение выражения: a) \(7\frac{1}{8} \cdot 4\frac{3}{4} \cdot 8\); б) \(11\frac{1}{3} : \frac{4}{21} : 4\frac{1}{4}\); г) \(\frac{7}{8}:\frac{8}{9}:1\frac{1}{9}\);

a)

$$7\frac{1}{8} \cdot 4\frac{3}{4} \cdot 8 = \frac{57}{8} \cdot \frac{19}{4} \cdot 8 = \frac{57 \cdot 19 \cdot 8}{8 \cdot 4} = \frac{57 \cdot 19}{4} = \frac{1083}{4} = 270\frac{3}{4}$$

б)

$$11\frac{1}{3} : \frac{4}{21} : 4\frac{1}{4} = \frac{34}{3} : \frac{4}{21} : \frac{17}{4} = \frac{34}{3} \cdot \frac{21}{4} \cdot \frac{4}{17} = \frac{34 \cdot 21 \cdot 4}{3 \cdot 4 \cdot 17} = \frac{2 \cdot 17 \cdot 7 \cdot 3 \cdot 4}{3 \cdot 4 \cdot 17} = 2 \cdot 7 = 14$$

г)

$$\frac{7}{8}:\frac{8}{9}:1\frac{1}{9} = \frac{7}{8} : \frac{8}{9} : \frac{10}{9} = \frac{7}{8} \cdot \frac{9}{8} \cdot \frac{9}{10} = \frac{7 \cdot 9 \cdot 9}{8 \cdot 8 \cdot 10} = \frac{567}{640}$$

Ответ: a) \(270\frac{3}{4}\); б) 14; г) \(\frac{567}{640}\).