jmc

Dividing by $400,$ we get $$\frac{x^2}{20^2}+\frac{y^2}{10^2}=1.$$Thus, the length of the major and minor axes are $2\cdot 20=40$ and $2\cdot 10=20,$ respectively. Then the distance between the foci of the ellipse is $\sqrt{40^2-20^2}=\boxed{20\sqrt{3}}.$