a) \( \frac{1}{3} + \frac{1}{6} = \frac{2}{6} + \frac{1}{6} = \frac{3}{6} = \frac{1}{2} \), б) \( \frac{1}{3} + \frac{4}{5} = \frac{5}{15} + \frac{12}{15} = \frac{17}{15} \), в) \( \frac{5}{9} + \frac{1}{6} = \frac{30}{54} + \frac{9}{54} = \frac{39}{54} = \frac{13}{18} \)