$\dfrac{5}{1.6} + \dfrac{5}{6.11} + ..... + \dfrac{5}{(5x+1)(5x+6)}= \dfrac{2019}{2020}$
$⇔ 1 - \dfrac{1}{5x+6} = \dfrac{2019}{2020}$
$⇔ \dfrac{1}{5x+6} = \dfrac{1}{2020}$
$⇔ 5x + 6 = 2020$
$⇔ 5x = 2014$
$⇔ x = \dfrac{2014}{5}$
Vậy $x= \dfrac{2014}{5}$