$Q = \dfrac{3}{1.2} + \dfrac{3}{2.3} + \dfrac{3}{3.4} + ... + \dfrac{3}{20.21}$
$Q = \dfrac{1}{1} ( \dfrac{3}{1.2} + \dfrac{3}{2.3} + \dfrac{3}{3.4} + ... + \dfrac{3}{20.21} )$
$ Q = 3 ( \dfrac{1}{1.2} + \dfrac{1}{2.3} + ... + \dfrac{1}{20.21} )$
$ Q = 3 ( 1 - \dfrac{1}{2} + \dfrac{1}{2} - \dfrac{1}{3} + ... + \dfrac{1}{20} - \dfrac{1}{21} )$
$Q = 3 ( 1 - \dfrac{1}{21} )$
$ Q = 3 . \dfrac{20}{21}$
$ Q = \dfrac{60}{21} = \dfrac{20}{7}$
