a) $\dfrac{3}{4}+\dfrac{5}{-6}-\dfrac{-1}{20}$
$ = \dfrac{18-20}{24} +\dfrac{1}{20}$
$ = \dfrac{-1}{12} + \dfrac{1}{20}$
$ = \dfrac{-1}{30}$
b) $\dfrac{-5}{7} :\dfrac{13}{14}+\dfrac{-8}{7} : \dfrac{13}{14} -(-1)^{2020}$
$ = ( \dfrac{-5-8}{7}): \dfrac{13}{14} -1$
$ = \dfrac{-13}{7}.\dfrac{14}{13} -1$
$ = -2-1=-3$
c) $|-\dfrac{3}{5}| + 60$%$ : (11\dfrac{4}{51}-12\dfrac{4}{51}) + (\dfrac{-2}{3})^3$
$ = \dfrac{3}{5} + \dfrac{60}{100} : (-1) + \dfrac{-8}{9} $
$ = \dfrac{3}{5} + \dfrac{-3}{5}+\dfrac{-8}{9}$
$ = \dfrac{-8}{9}$
