Tính rồi rút gọn.

\(\frac{{11}}{{24}}\) +\(\frac{{15}}{{24}}\) = \(\frac{{11 + 15}}{{24}}\)= \(\frac{{26}}{{24}}\)= \(\frac{{13}}{{12}}\)

\(\frac{{21}}{{20}}\) - \(\frac{{13}}{{20}}\) = \(\frac{{21 - 13}}{{20}}\)= \(\frac{8}{{20}}\)= \(\frac{2}{5}\)

 \(\frac{1}{3}\)  + \(\frac{7}{{15}}\) = \(\frac{5}{{15}}\)  + \(\frac{7}{{15}}\)= \(\frac{{12}}{{15}}\)= \(\frac{4}{5}\) 

\(\frac{{33}}{{35}}\) - \(\frac{{18}}{{35}}\) = \(\frac{{33 - 18}}{{35}}\)= \(\frac{{15}}{{35}}\)= \(\frac{3}{7}\)