$...=2.(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+....+\frac{1}{132})$$=2.(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{11.12})$
$=2.(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{11}-\frac{1}{12})$
$=2.(\frac{1}{4}-\frac{1}{12})=\frac{1}{3}$