"BTW, I've been to Manhattan and noticed that there are remarkably few gas stations there for a city of its size, so whatever estimate you come up with will pro

"BTW, I've been to Manhattan and noticed that there are remarkably few gas stations there for a city of its size, so whatever estimate you come up with will probably be very, very wrong." Yes, and that's precisely the sort of thing you're supposed to take into account. Again, you're looking to get in the right ballpark. They're not looking for precision. Asking "what steps would you take" wouldn't be quite the same question. They want you to actually take those steps to come up with a number. Ultimately, I'm not necessarily defending estimation questions. I'm just saying that they are, in fact, asked because interviewers don't consider them to be brainteasers.