$a)\frac{1}{4}+\frac{1}{5}=\frac{5}{20}+\frac{4}{20}=\frac{9}{20}$

$b)\frac{2}{3}+\frac{5}{7}=\frac{14}{21}+\frac{15}{21}=\frac{29}{21}=1\frac{8}{21}$  

$c)1\frac{1}{10}+5\frac{4}{5}=1\frac{1}{10}+5\frac{8}{10}=6\frac{9}{10}$

$d)\frac{5}{6}+3\frac{11}{12}=\frac{10}{12}+3\frac{11}{12}=3\frac{21}{12}=3\frac{7}{4}=4\frac{3}{4}$

$e)\frac{1}{4}+\frac{1}{5}+\frac{3}{10}=\frac{5}{20}+\frac{4}{20}+\frac{6}{20}=\frac{15}{20}=\frac{3}{4}$

$f)5+7\frac{8}{9}+4\frac{1}{2}=12\frac{8}{9}+4\frac{1}{2}=12\frac{16}{18}+4\frac{9}{18}=16\frac{25}{18}=17\frac{7}{18}$