\[ \frac{15}{14} + \frac{5}{14} = \frac{15+5}{14} = \frac{20}{14} \]
\[ \frac{13}{14} + \frac{8}{14} - \frac{20}{14} = \frac{13+8-20}{14} \]
\[ = \frac{21-20}{14} = \frac{1}{14} \]
Ответ: \( \frac{1}{14} \).