\(\left(x-3\right):\dfrac{4}{5}=20:\left(x-3\right)\)
\(\Leftrightarrow\left(x-3\right).\dfrac{5}{4}-\left(x-3\right).\dfrac{1}{20}=0\)
\(\Leftrightarrow\left(x-3\right)\left(\dfrac{5}{4}-\dfrac{1}{20}\right)=0\)
\(\Leftrightarrow\left(x-3\right).\dfrac{6}{5}=0\)
\(\Leftrightarrow x-3=0\)
\(\Leftrightarrow x=3\)