a) $$ \frac{1}{8} + \frac{5}{8} = \frac{1+5}{8} = \frac{6}{8} = \frac{3}{4} $$
б) $$ \frac{7}{15} - \frac{2}{15} = \frac{7-2}{15} = \frac{5}{15} = \frac{1}{3} $$