In a group of 100 persons 75 speak English and 40 speak Hindi. Each person speaks at least one of the two languages. If the number of persons who speak only English is $a$ and the number of persons who speaks only Hindi is $\beta$, then the eccentricity of the ellipse $25(\beta^2 x^2 + a^2 y^2) = a^2 \beta^2$ is