$a)\frac{7}{20}+\frac{1}{2}=\frac{7}{20}+\frac{10}{20}=\frac{17}{20}$

$b)\frac{3}{16}+\frac{1}{4}=\frac{3}{16}+\frac{4}{16}=\frac{7}{16}$

$c)\frac{5}{6}+\frac{1}{12}=\frac{10}{12}+\frac{1}{12}=\frac{11}{12}$

$d)\frac{11}{21}+\frac{2}{7}=\frac{11}{21}+\frac{6}{21}=\frac{17}{21}$