Answer
$\frac{x^2}{400}+\frac{y^2}{144}=1$,
$10.4\ ft$
Work Step by Step
1. Start from the standard equation $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$. Based on the given information, we have $2a=40, a=20$ and $b=12$. Thus the equation is $\frac{x^2}{400}+\frac{y^2}{144}=1$.
2. Let $x=10\ ft$, use it in the equation, we have $\frac{10^2}{400}+\frac{y^2}{144}=1$, thus $y^2=144(1-0.25)\approx10.4\ ft$
