31) \(\frac{21}{27}-\frac{2}{3}\)

\(\frac{21}{27}=\frac{7}{9}\), а \(\frac{2}{3}=\frac{6}{9}\).

\(\frac{7}{9}-\frac{6}{9}=\frac{1}{9}\).

Ответ: \(\frac{1}{9}\)