2) \((\frac{5}{6}-\frac{3}{8}):\frac{3}{4}-(\frac{3}{8}+\frac{7}{20}):1\frac{9}{20}\)

\[\left(\frac{5}{6}-\frac{3}{8}\right):\frac{3}{4}-\left(\frac{3}{8}+\frac{7}{20}\right):1\frac{9}{20}\]

\[\frac{5}{6}-\frac{3}{8}=\frac{20}{24}-\frac{9}{24}=\frac{11}{24}\]

\[\frac{11}{24}:\frac{3}{4}=\frac{11}{24}\cdot\frac{4}{3}=\frac{11}{18}\]

\[\frac{3}{8}+\frac{7}{20}=\frac{15}{40}+\frac{14}{40}=\frac{29}{40}\]

\[1\frac{9}{20}=\frac{29}{20},\qquad \frac{29}{40}:\frac{29}{20}=\frac{1}{2}\]

\[\frac{11}{18}-\frac{1}{2}=\frac{11}{18}-\frac{9}{18}=\frac{1}{9}\]

Ответ: \(\frac{1}{9}\).