I think this particular question (a so-called Fermi problem) is a perfectly legitimate interview question. Being able to do "back-of-the-envelope calculations"

I think this particular question (a so-called Fermi problem) is a perfectly legitimate interview question. Being able to do "back-of-the-envelope calculations" is a necessary or helpful skill for many technical and analytical jobs. It's not about getting the right answer, it's about the candidate's process to arrive at the answer.