$ B = \dfrac{5}{18.21} + \dfrac{5}{21.24} + \dfrac{5}{24.27} + ... + \dfrac{5}{123.126} $
$B = \dfrac{5}{3} ( \dfrac{3}{18.21} + \dfrac{3}{21.24} + \dfrac{3}{24.27} + ... + \dfrac{3}{123.126} )$
$ B = \dfrac{5}{3} ( \dfrac{1}{18} - \dfrac{1}{21} + \dfrac{1}{21} - \dfrac{1}{24} + ... + \dfrac{1}{123} - \dfrac{1}{126} )$
$B = \dfrac{5}{3} ( \dfrac{1}{18} - \dfrac{1}{126} )$
$B = \dfrac{5}{3} . \dfrac{6}{126} $
$B = \dfrac{5}{3} . \dfrac{1}{21}$
$ B = \dfrac{5}{36}$