Ok, so when they ask that question, they are looking for two things.
First your initial response. Will you scream out loud and say this is absurd, OR will you sit back and ponder for a few seconds then say that it is a very interesting question and state your answer.
Secondly, they are not looking for an actual number, or the correct mathematical formula to derive the answer. They are just checking to see if you have any rationality when thinking analytically.
For example in regards to the gas station question, I would start of with....
Well firstly I would contact the DMV and ask for the number of registered cars for a specific region, then I would contact the anothe government agency which holds records for the number of companies that operate gas stations in that specific area, and then acquire information from from some more reliable agencies to double check my estimates ... etc etc
Just my two cents.... btw this was advice given to me by my interviewER
When in Medical School and in the medical profession, you are posed with questions from patients, coworkers, professors, atetendings... You might not know the answer off hand let alone any idea vaguely close to what the question is asking but if you know where to look it up, you are safe! You will have to look them up in RELIABLE sources, journals, PDRs etc, to make sure you are giving an accurate answer.