Tính.

\(\frac{{39}}{{45}}\) + \(\frac{{13}}{{15}}\) = \(\frac{{13}}{{15}}\) + \(\frac{{13}}{{15}}\) = \(\frac{{26}}{{15}}\)

\(\frac{{21}}{{18}}\) – \(\frac{{10}}{9}\) = \(\frac{{21}}{{18}}\) – \(\frac{{20}}{{18}}\) = \(\frac{1}{{18}}\)

9 × \(\frac{5}{{63}}\) = \(\frac{5}{7}\)

\(\frac{1}{7}\) : \(\frac{5}{{28}}\) = \(\frac{1}{7}\) × \(\frac{{28}}{5}\) = \(\frac{4}{5}\)