[tex]\displaystyle\bf\\\frac{40x^{2} -21xy}{xy} =\frac{40x^{2} }{xy} -\frac{21xy}{xy} =\frac{40x}{y} -21\\\\\\\frac{x}{y} =20 \ \ \Rightarrow \ \ x=20y\\\\\\\frac{40x}{y} -21=\frac{40\cdot 20y}{y} -21=800-21=779\\\\\\Otvet \ : \ 779[/tex]