A ruler won't help you here

By definition, the directrices of an ellipse have the formula $x = \pm \dfrac{a}{e}$ , where $e$ is eccentricity. We know that $e = \dfrac{c}{a}$ , where $a$ is half the total length of major axis and $c$ is distance to focus, so we just needed to compute $\dfrac{a^{2}}{c}$ , or $\dfrac{25}{4}$ , and when we compute what the question asked for, or $100*\dfrac{25}{4}$ , we get $\boxed{625}$ .