This is the way I would answered in an interview as well. I guess it heavily relies on the type of town you are from. Even smaller towns in remote areas tend to have a gas station. But I think whats important is not your answer, but how you came to the number, and 3x off is actually not bad for something you don't know anything about.

I used the same method but of course had different numbers to plug in: 4 gas stations (I am probably forgetting a few in parts of town I didn't frequent) in a town of roughly 10,000 people.

Maybe you'd get a better figure going by number of pumps per person and then <strike>making up</strike> coming up with an average number of pumps per station. For example, (I'm guessing that) the lone gas station in a 200 person down doesn't have a lot of pumps.

I came to 200,000 by guessing how many were in an area served by one high school, and trying to remember how many high schools are in the US. Odd, but I figured a high school grouping would be some indicator of economic activity and thus civilian gas needs.

Funny, I came to the same estimate using my town of 30,000 with approximately 30 gas stations.