\(a.\dfrac{x}{5}-\dfrac{16}{40}=0.\\ \Leftrightarrow\dfrac{x}{5}-\dfrac{2}{5}=0.\\ \Leftrightarrow\dfrac{x}{5}=\dfrac{2}{5}.\\ \Rightarrow x=2.\\ b.\dfrac{y}{25}-\dfrac{-2}{5}=0.\\ \Leftrightarrow\dfrac{y}{25}=\dfrac{-2}{5}.\\ \Leftrightarrow\dfrac{y}{25}=\dfrac{-10}{25}.\\ \Leftrightarrow y=-10.\)

\(c.\dfrac{-5}{y}=\dfrac{20}{28}.\\ \Leftrightarrow\dfrac{-5}{y}=\dfrac{5}{7}.\\ \Rightarrow y=-7.\\ d.\dfrac{4}{-x}=\dfrac{8}{6}\left(x\ne0\right).\\ \Leftrightarrow\dfrac{4}{-x}=\dfrac{4}{3}.\\ \Leftrightarrow x=-3\left(TM\right).\)