а) 7/8 - 5/6 * 9/20 = б) 2/9 + 4/7 : 9/28 = в) 2 5/8 : 3/4 + 1/2 = г) 1 1/4 + 2/7 - 23/28 = д) (17/15 - 1/12) * 20/3 = е) 21/4 : 49/64 * 7/8 = ж) 7/2 : (7/8 - 3/4) = з) 45/8 * 6/5 : 9/4 = и) (1 2/7 - 1/2) * 28 = к) 1 - 3/7 : 15/28 = л) 9/14 : 5/2 - 4/35 = м) 32/7 : 16/35 * 4/5 =

Решаем:

а) \(\frac{7}{8} - \frac{5}{6} \cdot \frac{9}{20} = \frac{7}{8} - \frac{5 \cdot 9}{6 \cdot 20} = \frac{7}{8} - \frac{45}{120} = \frac{7}{8} - \frac{3}{8} = \frac{4}{8} = \frac{1}{2}\) б) \(\frac{2}{9} + \frac{4}{7} : \frac{9}{28} = \frac{2}{9} + \frac{4}{7} \cdot \frac{28}{9} = \frac{2}{9} + \frac{4 \cdot 28}{7 \cdot 9} = \frac{2}{9} + \frac{4 \cdot 4}{9} = \frac{2}{9} + \frac{16}{9} = \frac{18}{9} = 2\) в) \(2\frac{5}{8} : \frac{3}{4} + \frac{1}{2} = \frac{21}{8} : \frac{3}{4} + \frac{1}{2} = \frac{21}{8} \cdot \frac{4}{3} + \frac{1}{2} = \frac{21 \cdot 4}{8 \cdot 3} + \frac{1}{2} = \frac{7 \cdot 1}{2 \cdot 1} + \frac{1}{2} = \frac{7}{2} + \frac{1}{2} = \frac{8}{2} = 4\) г) \(1\frac{1}{4} + \frac{2}{7} - \frac{23}{28} = \frac{5}{4} + \frac{2}{7} - \frac{23}{28} = \frac{5 \cdot 7}{4 \cdot 7} + \frac{2 \cdot 4}{7 \cdot 4} - \frac{23}{28} = \frac{35}{28} + \frac{8}{28} - \frac{23}{28} = \frac{35 + 8 - 23}{28} = \frac{20}{28} = \frac{5}{7}\) д) \((\frac{17}{15} - \frac{1}{12}) \cdot \frac{20}{3} = (\frac{17 \cdot 4}{15 \cdot 4} - \frac{1 \cdot 5}{12 \cdot 5}) \cdot \frac{20}{3} = (\frac{68}{60} - \frac{5}{60}) \cdot \frac{20}{3} = \frac{63}{60} \cdot \frac{20}{3} = \frac{63 \cdot 20}{60 \cdot 3} = \frac{21 \cdot 1}{3 \cdot 1} = 7\) е) \(\frac{21}{4} : \frac{49}{64} \cdot \frac{7}{8} = \frac{21}{4} \cdot \frac{64}{49} \cdot \frac{7}{8} = \frac{21 \cdot 64 \cdot 7}{4 \cdot 49 \cdot 8} = \frac{3 \cdot 16 \cdot 1}{1 \cdot 7 \cdot 1} = \frac{48}{7} = 6\frac{6}{7}\) ж) \(\frac{7}{2} : (\frac{7}{8} - \frac{3}{4}) = \frac{7}{2} : (\frac{7}{8} - \frac{3 \cdot 2}{4 \cdot 2}) = \frac{7}{2} : (\frac{7}{8} - \frac{6}{8}) = \frac{7}{2} : \frac{1}{8} = \frac{7}{2} \cdot 8 = \frac{7 \cdot 8}{2} = 7 \cdot 4 = 28\) з) \(\frac{45}{8} \cdot \frac{6}{5} : \frac{9}{4} = \frac{45}{8} \cdot \frac{6}{5} \cdot \frac{4}{9} = \frac{45 \cdot 6 \cdot 4}{8 \cdot 5 \cdot 9} = \frac{5 \cdot 3 \cdot 1}{2 \cdot 1 \cdot 1} = \frac{15}{2} = 7\frac{1}{2}\) и) \((1\frac{2}{7} - \frac{1}{2}) \cdot 28 = (\frac{9}{7} - \frac{1}{2}) \cdot 28 = (\frac{9 \cdot 2}{7 \cdot 2} - \frac{1 \cdot 7}{2 \cdot 7}) \cdot 28 = (\frac{18}{14} - \frac{7}{14}) \cdot 28 = \frac{11}{14} \cdot 28 = \frac{11 \cdot 28}{14} = 11 \cdot 2 = 22\) к) \(1 - \frac{3}{7} : \frac{15}{28} = 1 - \frac{3}{7} \cdot \frac{28}{15} = 1 - \frac{3 \cdot 28}{7 \cdot 15} = 1 - \frac{1 \cdot 4}{1 \cdot 5} = 1 - \frac{4}{5} = \frac{5}{5} - \frac{4}{5} = \frac{1}{5}\) л) \(\frac{9}{14} : \frac{5}{2} - \frac{4}{35} = \frac{9}{14} \cdot \frac{2}{5} - \frac{4}{35} = \frac{9 \cdot 2}{14 \cdot 5} - \frac{4}{35} = \frac{9}{7 \cdot 5} - \frac{4}{35} = \frac{9}{35} - \frac{4}{35} = \frac{5}{35} = \frac{1}{7}\) м) \(\frac{32}{7} : \frac{16}{35} \cdot \frac{4}{5} = \frac{32}{7} \cdot \frac{35}{16} \cdot \frac{4}{5} = \frac{32 \cdot 35 \cdot 4}{7 \cdot 16 \cdot 5} = \frac{2 \cdot 5 \cdot 4}{1 \cdot 1 \cdot 5} = \frac{40}{5} = 8\)