18. \(\frac{3}{7}+\frac{2}{5}\)

Общий знаменатель — 35: \(\frac{3}{7}=\frac{15}{35}\), \(\frac{2}{5}=\frac{14}{35}\).

\(\frac{15}{35}+\frac{14}{35}=\frac{29}{35}\).

Ответ: \(\frac{29}{35}\)