$A = \dfrac{6}{15.18} + \dfrac{6}{18.21} + ... + \dfrac{6}{87.90}$
$A = \dfrac{6}{3} ( \dfrac{3}{15.18} + \dfrac{6}{18.21} + ... + \dfrac{3}{87.90} )$
$A = 2 ( \dfrac{1}{15} - \dfrac{1}{18} + \dfrac{1}{18} - \dfrac{1}{21} + ... + \dfrac{1}{87} - \dfrac{1}{90} )$
$A = 2 ( \dfrac{1}{15} - \dfrac{1}{90} )$
$A = 2 . \dfrac{1}{18}$
$A = \dfrac{2}{18} = \dfrac{1}{9}$
