a) \frac{1}{6} + \frac{3}{7} = \frac{7}{42} + \frac{18}{42} = \frac{25}{42}
б) \frac{3}{8} + \frac{7}{16} = \frac{6}{16} + \frac{7}{16} = \frac{13}{16}
в) \frac{7}{15} - \frac{3}{20} = \frac{28}{60} - \frac{9}{60} = \frac{19}{60}
г) \frac{1}{8} + \frac{5}{6} - \frac{7}{12} = \frac{3}{24} + \frac{20}{24} - \frac{14}{24} = \frac{9}{24} = \frac{3}{8}