A Fermi problem is not a brain teaser in the colloquial sense, it's more of an approximation game. If you looked up an answer a week ago and memorized it, then

A Fermi problem is not a brain teaser in the colloquial sense, it's more of an approximation game. If you looked up an answer a week ago and memorized it, then regurgitated it, you'd still get the question "wrong". No one expects you to come up with the right number of windows, they expect you to get somewhere plausible from scratch. Asking someone how many windows are in NYC is not fundamentally different than asking someone how many blades of grass are on a soccer field. You can know almost nothing and still arrive at the correct order of magnitude -- all it tests is someone's ability to do napkin math and break open-ended problems into manageable parts. Is this the right thing to test? That depends on what you're hiring someone for.