Решение:

1. a) $$(\frac{1}{2} + \frac{1}{5}) : 2 - \frac{3}{10} = (\frac{5}{10} + \frac{2}{10}) : 2 - \frac{3}{10} = \frac{7}{10} : 2 - \frac{3}{10} = \frac{7}{20} - \frac{3}{10} = \frac{7}{20} - \frac{6}{20} = \frac{1}{20}$$

2. б) $$(\frac{3}{4} - \frac{5}{12}) \cdot 8 + \frac{1}{18} = (\frac{9}{12} - \frac{5}{12}) \cdot 8 + \frac{1}{18} = \frac{4}{12} \cdot 8 + \frac{1}{18} = \frac{1}{3} \cdot 8 + \frac{1}{18} = \frac{8}{3} + \frac{1}{18} = \frac{48}{18} + \frac{1}{18} = \frac{49}{18} = 2\frac{13}{18}$$

3. в) $$(\frac{7}{15} + \frac{4}{5}) : 3 - \frac{1}{9} = (\frac{7}{15} + \frac{12}{15}) : 3 - \frac{1}{9} = \frac{19}{15} : 3 - \frac{1}{9} = \frac{19}{45} - \frac{1}{9} = \frac{19}{45} - \frac{5}{45} = \frac{14}{45}$$

4. г) $$(\frac{3}{7} - \frac{1}{21}) \cdot 14 - \frac{1}{6} = (\frac{9}{21} - \frac{1}{21}) \cdot 14 - \frac{1}{6} = \frac{8}{21} \cdot 14 - \frac{1}{6} = \frac{8}{3} \cdot 2 - \frac{1}{6} = \frac{16}{3} - \frac{1}{6} = \frac{32}{6} - \frac{1}{6} = \frac{31}{6} = 5\frac{1}{6}$$