a)

$\frac{1}{3}+\frac{3}{4}=\frac{4}{12}+\frac{9}{12}=\frac{13}{12}=1\frac{1}{12}$

$\frac{2}{3}+\frac{4}{7}=\frac{14}{21}+\frac{12}{21}=\frac{26}{21}=1\frac{5}{21}$

$\frac{5}{6}+\frac{7}{9}=\frac{15}{18}+\frac{14}{18}=\frac{29}{18}=1\frac{11}{18}$

$\frac{5}{8}+\frac{3}{10}=\frac{25}{40}+\frac{12}{40}=\frac{37}{40}$