Apparently some people take whatever warm body shows up.
It would only be valid not to hire them if they have already taken measurements and noted them down. If they can't do the calculation to convert their measurements to square footage and then to linear feet of board (with margins to allow for different board widths) - that'd be something different.
Even there I would argue one could still say "I don't know", simply because you haven't specified how you want the flooring oriented per room/area (and if it goes on the bias, it gets tricky quick), or if there is any kind of special patterns or inlays you want, etc.
Until you have the full specification at hand, you can't really give more than a ballpark answer, which in the case of flooring calcs could be wildly off (agile doesn't really work for real-world engineering like it can for software).
Likewise, for the original question, I'd expect some sort of discussion or nuanced answer. Something like, "Well, most modern CPUs are rated at the GHz, so on the basic level, we can say on the order of billions of operations - but we're getting a lot more cores, and given SIMD and such, you could actually go an order of magnitude or two over that, in theory. But if you are doing something that takes lots of clock cycles, it could drop down into 100s of millions, I suppose."
What you don't want to hear is a deranged ranting, like some people are doing in this very thread, or "I dunno" and a blank stare.
And again, none of that knowledge is relevant or required to be very competent at web development.
The equivalent question to a web developer would be how many lines of code would it take to make a bare bones site with a calculator in it? 1,000? 10,000?, 100,000? Options A and possibly B (with explanation would be acceptable).
Your question (although you don't seem to feel this way) is the equivalent of asking a carpenter: How many Watts of power will you consume while finishing this room? A carpenter who knows that may show lots of curiosity about their tools, but there are also plenty of skilled carpenters who don't know the answer to that.
Your interview question is more asking "How similar is this person to how I approach things?" than "How skilled is this person at doing their job?"