It's actually a tricky problem to think clearly about unless you start enumerating the cases, or get pen and paper out to draw some boxes. I don't think it's completely trivial to verbalise. But I also don't think a competent programmer should fail to work it out.

I like the problem, and I'll probably use it in future :) It'll be a low bar for those times where you're getting the impression that a bigger task will be beyond the candidate, and you want to cut your losses early.

FWIW, I've seen similar complete failures to reason when applied to the even simpler problem of testing to see if a point is inside a rectangle. Some people seem to have a very underdeveloped one dimensional measure function in their brains.