Except the problem is (which I didn't state but pointed out) is that this isn't really a "problem." It's just math. That's it. If you want to know if I know how to multiply two numbers together, you might as well just ask me if I know what 2*2 is and move on to the next question.

It is a gotcha question. In fact, "log2 of 56 million" should probably be the best answer. It means you don't waste time brute forcing the problem and instead know exactly how to get the solution, but they wanted a specific answer that you compute by some heuristic. Why?

Either way, if a candidate misses such a question it shouldn't be a negative. Using such a question as a filter is a bit ridiculous.