a)    $\frac{1}{5}+\frac{3}{5}=\frac{4}{5}$

$\frac{4}{11}+\frac{7}{11}=\frac{11}{11}=1$

$b)1\frac{1}{6}+\frac{3}{6}=1\frac{4}{6}=1\frac{2}{3}$

$4\frac{3}{10}+2\frac{5}{10}=6\frac{8}{10}=6\frac{4}{5}$

$c)2\frac{4}{9}+\frac{5}{9}=2\frac{9}{9}=3$

$6\frac{5}{8}+2\frac{3}{8}=8\frac{8}{8}=9$