Word problem involving ellipse: what is the tallest truck that can pass under the overpass

[Felidaei]

The question asks, "A one-way road has an overpass in the form of a semi-ellipse, 15 ft high at the center, and 40 ft wide. Assuming a truck is 12 ft wide, what is the tallest truck that can pass under the overpass?"

I don't think this is a super complicated question yet it proves to be too confusing for my brain >_<
Any kind of help would be appreciated, thank you!

 

[skeeter]

Upper branch of the ellipse $\dfrac{x^2}{20^2}+\dfrac{y^2}{15^2}=1$ is ...

$y=15\sqrt{1-\dfrac{x^2}{20^2}}$

Now determine $y(6)$ ...