Estimating the number of filling stations nationally

Make a reasoned estimate of how many petrol stations there are in the country you are in.

For the UK, roughly 8,000 to 10,000. Interviewers want a clear chain: about $30$ million cars each filling up roughly weekly gives about $4$ million fills a day; a station serving around $500$ fills a day implies about $8{,}000$. State assumptions, sanity-check against local density, and quote a range rather than a precise number.