$M = \dfrac{3}{5.7}+ \dfrac{3}{7.9} + .... + \dfrac{3}{59.61}$
$⇔ M : 3 = \dfrac{1}{5.7}+ \dfrac{1}{7.9} + .... + \dfrac{1}{59.61}$
$⇔ M : 3 .2 = \dfrac{2}{5.7}+ \dfrac{2}{7.9} + .... + \dfrac{2}{59.61}$
$⇔M : \dfrac{3}{2} = \dfrac{1}{5} - \dfrac{1}{7} + \dfrac{1}{7} - \dfrac{1}{9} + .... + \dfrac{1}{59} - \dfrac{1}{61}$
$⇔ M : \dfrac{3}{2} = \dfrac{1}{5} - \dfrac{1}{61}$
$⇔ M : \dfrac{3}{2} = \dfrac{56}{305}$
$⇔ M =\dfrac{84}{305}$
