a) \( \dfrac{7}{9}+\dfrac{9}{10}\) \( =\dfrac{70}{90}+\dfrac{81}{90}=\dfrac{151}{90}\) ;
b) \( \dfrac{5}{6}+\dfrac{7}{8}\) \( =\dfrac{20}{24}+\dfrac{21}{24}=\dfrac{41}{24}\) ;
c) \( \dfrac{3}{5}+\dfrac{1}{2}+\dfrac{3}{10}= \dfrac{6}{10}+\dfrac{5}{10}+\dfrac{3}{10}\) \(=\dfrac{6+5+3}{10}=\dfrac{14}{10}=\dfrac{7}{5}\) .