a) $(\dfrac{-7}{5}+\dfrac{3}{8}):\dfrac{2019}{2020}+(\dfrac{-3}{5}+\dfrac{5}{8}):\dfrac{2019}{2020}$
$=(\dfrac{-7}{5}+\dfrac{3}{8}).\dfrac{2020}{2019}+(\dfrac{-3}{5}+\dfrac{5}{8}).\dfrac{2020}{2019}$
$=\dfrac{2020}{2019}.[(\dfrac{-7}{5}+\dfrac{3}{8})+(\dfrac{-3}{5}+\dfrac{5}{8})]$
$=\dfrac{2020}{2019}.[\dfrac{-7}{5}+\dfrac{3}{8}+\dfrac{-3}{5}+\dfrac{5}{8}]$
$=\dfrac{2020}{2019}.[(\dfrac{-7}{5}+\dfrac{-3}{5})+(\dfrac{3}{8}+\dfrac{5}{8})]$
$=\dfrac{2020}{2019}.[-2+1]$
$=\dfrac{2020}{2019}.(-1)=-\dfrac{2020}{2019}$
b) $(\dfrac{3}{5}+5\dfrac{2}{3}):\dfrac{2020}{2021}+(-5\dfrac{2}{3}+\dfrac{2}{5}):\dfrac{2020}{2021}$
$=(\dfrac{3}{5}+5\dfrac{2}{3}).\dfrac{2021}{2020}+(-5\dfrac{2}{3}+\dfrac{2}{5}).\dfrac{2021}{2020}$
$=\dfrac{2021}{2020}.[(\dfrac{3}{5}+5\dfrac{2}{3})+(-5\dfrac{2}{3}+\dfrac{2}{5}]$
$=\dfrac{2021}{2020}.[\dfrac{3}{5}+5\dfrac{2}{3}-5\dfrac{2}{3}+\dfrac{2}{5}]$
$=\dfrac{2021}{2020}.[(\dfrac{3}{5}+\dfrac{2}{5})+(5\dfrac{2}{3}-5\dfrac{2}{3})]$
$=\dfrac{2021}{2020}.[1-1]=\dfrac{2021}{2020}.0=0$
