13. Вычислите: \( \frac{25}{54} \div \left(\frac{7}{12} - \frac{13}{18}\right) + 2 \cdot 2 \frac{2}{3} \).

Решение:\( \frac{7}{12} - \frac{13}{18} = \frac{21}{36} - \frac{26}{36} = -\frac{5}{36} \). \( \frac{25}{54} \div -\frac{5}{36} = \frac{25}{54} \cdot -\frac{36}{5} = -\frac{900}{270} = -\frac{10}{3} \). \( 2 \cdot 2 \frac{2}{3} = 2 \cdot \frac{8}{3} = \frac{16}{3} \). Ответ: \( -\frac{10}{3} + \frac{16}{3} = \frac{6}{3} = 2 \).