139. Найдите значение выражения: а) $$2\frac{1}{7}:4\frac{1}{6}\cdot2\frac{1}{3}$$; б) $$(7\frac{1}{3}+2\frac{1}{4}):\frac{1}{4}-30\frac{5}{6}$$.

a) $$2\frac{1}{7}:4\frac{1}{6}\cdot2\frac{1}{3} = \frac{15}{7}:\frac{25}{6} \cdot \frac{7}{3} = \frac{15}{7} \cdot \frac{6}{25} \cdot \frac{7}{3} = \frac{15 \cdot 6 \cdot 7}{7 \cdot 25 \cdot 3} = \frac{3 \cdot 5 \cdot 2 \cdot 3 \cdot 7}{7 \cdot 5 \cdot 5 \cdot 3} = \frac{6}{5} = 1\frac{1}{5}$$

б) $$(7\frac{1}{3}+2\frac{1}{4}):\frac{1}{4}-30\frac{5}{6} = (\frac{22}{3}+\frac{9}{4}):\frac{1}{4}-\frac{185}{6} = (\frac{22 \cdot 4 + 9 \cdot 3}{12}):\frac{1}{4}-\frac{185}{6} = (\frac{88+27}{12}):\frac{1}{4}-\frac{185}{6} = \frac{115}{12}:\frac{1}{4}-\frac{185}{6} = \frac{115}{12} \cdot 4 - \frac{185}{6} = \frac{115}{3}-\frac{185}{6} = \frac{115 \cdot 2 - 185}{6} = \frac{230-185}{6} = \frac{45}{6} = \frac{15}{2} = 7\frac{1}{2}$$

Ответ: а) $$1\frac{1}{5}$$; б) $$7\frac{1}{2}$$