You are asked this in the first ten minutes of a consulting interview. There is no data in the room and no internet. The interviewer wants to watch you think, not to check your answer against a source.
Mumbai's population is roughly 2 crore (20 million).
Estimate the number of cups of chai sold in Mumbai on an ordinary weekday. Break the number down before you compute anything, state every assumption and why you chose it, do the arithmetic, then sanity-check the result.
100 points, 60% to pass.
Segment the population by chai behaviour rather than treating it as one block: office-goers who buy 2-3 cups from a tapri, households who make it at home (and so buy none), students, and travellers. Multiply segment size by cups bought per person per day. Exclude home-made chai explicitly — it is the single biggest swing factor and the most common thing candidates forget to separate.