а) $$ \frac{1}{16} + (\frac{3}{16} + \frac{1}{4}) = \frac{1}{16} + (\frac{3}{16} + \frac{4}{16}) = \frac{1}{16} + \frac{7}{16} = \frac{1+7}{16} = \frac{8}{16} = \frac{1}{2} $$
б) $$ \frac{1}{12} + \frac{1}{18} + \frac{5}{18} + \frac{3}{12} = \frac{1}{12} + \frac{3}{12} + \frac{1}{18} + \frac{5}{18} = \frac{1+3}{12} + \frac{1+5}{18} = \frac{4}{12} + \frac{6}{18} = \frac{1}{3} + \frac{1}{3} = \frac{1+1}{3} = \frac{2}{3} $$
в) $$ \frac{2}{5} + \frac{3}{4} - \frac{23}{20} = \frac{8}{20} + \frac{15}{20} - \frac{23}{20} = \frac{8+15-23}{20} = \frac{23-23}{20} = \frac{0}{20} = 0 $$