Market sizing questions are usually asked in consulting interviews but may be asked in IB or PE interviews since industry analysis is part of the job. It’s more important to explain the logic behind your assumptions and less important to get the right number.
We can make an estimate of what % of total cars in New York City are black. For example, 1 in 5 would be a reasonable estimate. Another way to figure out a reasonable percentage in an in-person interview is to look out the window and see how many black cars you see as a percentage of the total cars that you observe.
We can assume that the average household in New York City is 2 people, and with a population of 8 million, there would be 4 million (8 million / 2). households in New York City. We can then assume that half of the households have a car, which would be a total of 2 million cars (4 million / 2) in New York City. If 20% of those cars are black cars, then there would be 400,000 black cars (2 million * 20%) in New York City.
If we assume that the average car is worth $30,000, then the value of the black car market in New York City is $12M ($30,000 x 400,000).