$\frac{4}{15}+\frac{7}{15}=\frac{11}{15}$

$2\frac{1}{13}+\frac{7}{13}=2\frac{8}{13}$

$\frac{9}{20}+\frac{3}{20}=\frac{12}{20}=\frac{6}{10}=\frac{3}{5}$

$\frac{5}{11}+4\frac{3}{11}=4\frac{8}{11}$

$\frac{13}{25}+\frac{20}{25}=\frac{33}{25}=\frac{25+8}{25}=1\frac{8}{25}$

$3\frac{5}{31}+5\frac{17}{31}=8\frac{22}{31}$