This question takes many forms, but the premise of it is quite simple. It’s asking you to work through a mathematical problem, usually figuring out the number of an item in a certain place, or figuring out how much of something could potentially be sold somewhere. Here are some real examples from Glassdoor:

“How many piano tuners are in the city of Chicago?” (Quicken Loans)
“How many windows are in New York City, by you estimation?” (Petco)
“How many gas stations are there in the United States?” (Progressive)
The idea here is to put you in a situation where you can’t possibly know something off the top of your head, but to see you work through it anyway. That’s the trap, though. You don’t want to just give up and say, well, gee, I don’t know. As James Patounas, associate director and senior data analyst at Source One, puts it, “I have been asked something similar as well as asked something similar. I personally would not accept ‘you can’t really know’ as an answer; or, at least, I would not hire someone that thought this was a sufficient answer.”

He went on: “Mathematical modeling is typically an approximation of the real world. It is rarely an exact representation.”

Basically, you want to pull the data you do have, or at least can approximate, and work yourself through a solution. Let’s take the number of windows in New York City as an example for the sample answer below.

Note: Figures in this answer do not necessarily realistically reflect facts; they are approximations (there are actually 8.6 million people in NYC, according to 2017 data, for example).

I believe there are about 10 million people in New York, give or take a couple million. Assuming each of them lives in a residential building, with three rooms or more, if there were one window per room, that would make approximately 30 million windows. I’m making a few different assumptions that are probably inaccurate. For instance, that everyone lives alone and that the average size of their residences is just three rooms with one window per room. Obviously, there will be a lot of variations in reality. But I think, in terms of residences, 30 million windows could be close.

Then you’d have to take windows for businesses, subway rail cars, and personal vehicles. If the average subway car seats 1,000 people, with 1 window per 2 seats, that’s 500 windows per car. A little more math: I’d guess there are at least enough subway cars to support the whole population of New York: so 10 million divided by 1,000 comes out to 10,000. So there are another 5 million windows for subway cars. If half of all people own their own vehicle, that’s another six windows per person, so 30 million more windows. I’d guess there are at least 100,000 businesses with windows in NYC. Let’s just say for the sake of argument there’s an average of 10 windows each. That’s another million. I’m sure there’s way more than that.

Overall, we’re at 66 million windows (30,000,000 x 2 + 5,000,000 + 1,000,000). All of this pretty much hinges on how close I am to the actual population of New York City. Also, there are other places to find windows, such as busses or boats. But that’s a start.