Consider the situation where you want to put a camera in a car, looking towards the front of the car. A car is roughly $2\mathrm{.}5$m wide. You have a camera with a $1$cm wide sensor with $2000$ horizontal pixels.

If you want the car to be at least $50$ pixels wide at $100$m$,$ what does your focal length need to be$?$ Give the answer in mm$.$ Be sure to identify which equation is relevant to the question before starting.