Найдите значение выражения: а) 7/20-(9/35-3/28) б) (21/22-5/11) - (22/39-3/13)

Решите задачу:

$\frac{7}{20} -( \frac{9}{35} - \frac{3}{28} )= \frac{7}{20} -( \frac{36-15}{140}) = \frac{7}{20} - \frac{21}{140} = \frac{7}{20} - \frac{3}{20} = \frac{4}{20} = \frac{1}{5} \\ \\ ( \frac{21}{22} - \frac{5}{11} )-( \frac{22}{39} - \frac{3}{13} )=( \frac{21-10}{22} )-( \frac{22-9}{39} )= \frac{11}{22} - \frac{13}{39} = \frac{1}{2} - \frac{1}{3} = \frac{3-2}{6} = \frac{1}{6}$