For a general-purpose developer interview question, I think this relies a little too heavily on domain-specific knowledge to be generally useful.

Personally, while I was quite decent at algebra and geometry in university, my skills have atrophied significantly, to the extent that I really didn't know where to start with this problem. I don't think it represents what most programmers do on a day-to-day basis, and someone with a strong math background would probably be far more capable of answering this, despite not necessarily being a better developer.

If this sort of thing represents what developers at your company would likely be doing on a daily basis, I think it could be a relevant question. Otherwise, I think a question like this is very likely to inadvertently screen out good developers.