a)

$\frac{1}{13}+\frac{5}{13}=\frac{6}{13}$

$\frac{7}{31}+\frac{19}{31}=\frac{26}{31}$

$\frac{7}{9}+\frac{19}{27}=\frac{21}{27}+\frac{19}{27}=\frac{40}{27}=1\frac{13}{27}$

$\frac{1}{25}+\frac{13}{15}=\frac{3}{75}+\frac{65}{75}=\frac{68}{75}$