$125\%.(-\dfrac{1}{2})^2:(1\dfrac{5}{16}-1,5)+2021^0$
$=\dfrac{5}{4}.\dfrac{1}{4}:(\dfrac{21}{16}-\dfrac{3}{2})+1$
$=\dfrac{5}{4}.\dfrac{1}{4}:(\dfrac{21}{16}-\dfrac{24}{16})+1$
$=\dfrac{5}{4}.\dfrac{1}{4}:\dfrac{-3}{16}+1$
$=\dfrac{5}{16}:\dfrac{-3}{16}+1$
$=\dfrac{5}{16}.\dfrac{-16}{3}+1$
$=\dfrac{-5}{3}+1$
$=\dfrac{-5}{3}+\dfrac{3}{3}$
$=-\dfrac{2}{3}$