You think those are the only confounding factors? What about tolerances so that no balls will be damaged in transit? Or is there an acceptable loss rate? How are the balls to be loaded into the trailer? Surely this is not a special ping-pong ball delivery trailer with a feeder and drainage pipe, so how will they be boxed? Do human factors affect the box size, and as a result, the number of balls that can fit? Will the trailers be stacked, and does this alter their capacity? If we don't have to box the balls, and they are stored tightly, could they settle in such a way as to become jammed, risking loss during unloading (tension is relieved, and they burst out the trailer)? Ping-pong balls are spherical, and the geometry of their fit is not as simple as that of rectangular objects, too. Are you expecting the candidate to solve a non-trivial geometry problem in a high-pressure situation? Why? Will the balls be transported between substantially different air pressures or temperatures? Is this important for ping-pong balls?
See how this could coax a poor performance out of an inquisitive, somewhat introverted candidate, rendering a false negative?
Beyond that, there's the more important adversarial tone: As an employee, will the smallest misstep result in termination? What if I have a problem that cannot be easily solved with a search engine? Is this a cooperative environment where I will feel somewhat secure as an employee, or is my status fragile? Is Mike a mercurial person who gives his employees brain-teasers with job performance implications?