How do you respond to a brain teaser like 'How many gas stations are there in Frankfurt?'

Market-size out loud.

1. Estimate population and commuters.
2. Estimate number of cars.
3. Estimate fuel visits per car per month.
4. Estimate visits a gas station can serve.
5. Divide demand by capacity.

'Frankfurt has around 750k residents, plus commuters, so maybe 1.1-1.2m daily population. If there are roughly 350k-450k cars linked to that demand and each refuels 2-3 times per month, that is around 800k-1.2m visits per month. If one station serves 10k-15k visits per month, that implies roughly 60-100 stations. I would sanity-check against city density and highway stations.'

structure, assumptions, and sanity checks. Exact answer is less important.