\(a.\frac{2}{5}+\frac{3}{6}=\frac{2}{5}+\frac{1}{2}=\frac{4}{10}+\frac{5}{10}=\frac{9}{10}.\)
\(b.\frac{21}{30}-\frac{9}{30}=\frac{21-9}{30}=\frac{12}{30}=\frac{2}{5}\)
\(c.\frac{5}{8}\cdot\frac{4}{7}=\frac{5\cdot4}{8\cdot7}=\frac{5\cdot1}{2\cdot7}=\frac{5}{14}\)
\(d.\frac{7}{9}:\frac{11}{6}=\frac{7}{9}\cdot\frac{6}{11}=\frac{7\cdot6}{9\cdot11}=\frac{7\cdot2}{3\cdot11}=\frac{14}{33}\)