7. Найдите значение выражения \(\left(\frac{1}{18}+\frac{1}{54}\right):\frac{8}{27}\).

\[\frac{1}{18}+\frac{1}{54}=\frac{3}{54}+\frac{1}{54}=\frac{4}{54}=\frac{2}{27}.\]

\[\frac{2}{27}:\frac{8}{27}=\frac{2}{27}\cdot\frac{27}{8}=\frac{1}{4}.\]

Ответ: \(\frac{1}{4}\).