Выполните действие: 7 16 a) 8 21; 8 4 6) 125 111 5 17 B) 17 18; 7 г) 5.315.

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

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

в) $$ 5\frac{5}{7} \cdot 1\frac{17}{18} = \frac{40}{7} \cdot \frac{35}{18} = \frac{40 \cdot 35}{7 \cdot 18} = \frac{2 \cdot 20 \cdot 7 \cdot 5}{7 \cdot 2 \cdot 9} = \frac{100}{9} = 11\frac{1}{9} $$.

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

Ответ: а) $$ \frac{2}{3} $$, б) $$1\frac{4}{5}$$, в) $$11\frac{1}{9}$$, г) $$17\frac{1}{3}$$.