$a) \dfrac{2}{-5} + \dfrac{2019}{2020}+ \dfrac{10}{25}$
$= \left ( \ {\dfrac{2}{-5}+\dfrac{10}{25}}\right)+ \dfrac{2019}{2020}$
$= \left ( \ {\dfrac{-10}{25}+\dfrac{10}{25}}\right)+ \dfrac{2019}{2020}$
$= 0 + \dfrac{2019}{2020}$
$= \dfrac{2019}{2020}$
$b) \dfrac{-5}{7} + \dfrac{3}{4} + \dfrac{-1}{5}+ \dfrac{-2}{7} + \dfrac{1}{4}$
$= \left ( \ {\dfrac{-5}{7}+\dfrac{-2}{7}}\right) + \left ( \ {\dfrac{3}{4}+\dfrac{1}{4}}\right) + \dfrac{-1}{5}$
$=-1+1+ \dfrac{-1}{5}$
$=0+ \dfrac{-1}{5}$
$=\dfrac{-1}{5}$