Related Rates (Shadows) Let me first apologize for what must be the $\frac{1}{\epsilon}$ th related rates question on this site, but I really have no idea how to tackle this. From Keisler's "Elementary Calculus": Problem 3.2.11. Answer: -120 mph I have an inkling that you will need to use the Pythagorean Theorem at some point, but what I don't see is where do we get enough information to even apply the theorem. Thank you very much.

Hint. The are two right triangles that are similar. One has a leg of length $10$ and the other has the corresponding leg of length $30-10=20=2\cdot 10$.