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.
For example, let’s assume the college has 4,000 undergraduate students. We can assume that only first years routinely buy breakfast at the cafeteria. This means 1 in 4 college students would routinely go to the cafeteria to buy breakfast, which is equal to 1,000 students (4,000 / 4).
We can assume that only 50% of first year students will get breakfast at the cafeteria that morning, which is equivalent to 500 students (1,000 * 50%). 1 in 2 of the students getting breakfast will order eggs, which is equivalent to 250 students (500 / 2). If each student ordering eggs orders an average of 1 egg, then 250 eggs (250 x 1) are ordered each morning at a college cafeteria.