Marina, 

I see Hagen already addressed your question directly below. 

I wanted to add that you should bring this attitude and these sorts of questions to how you interact with the interviewer through a market sizing exercise. 

At the end of the day, it's not the actual numerical answer that they care about, but rather how you think through the process, how you co-create an approach with the interviewer and finally, how you align with them on the core assumptions that will lead to the result. 

That means that you might have to ask clarifying questions upfront to clarify what is the scope of the market sizing question. Do that. It's great. 

If you're practicing for interviews at the moment, you might also find this helpful:

• Cheatsheet: The Must-Know Consulting Terms for Interviews

Best,
Cristian

Hi Marina,

I believe you are correct. The case states that SOME private schools are considering changing their textbooks. 

As such, the case should be providing the market share they represent.  In a real case, that would be a question to the interviewer that would make complete sense (and most likely they would provide you with a market share).

Of course, for this case, you will have to assume that the prompt is imprecise and all private schools are subject to the tender, otherwise the calculations would not make sense.

You should definitely ask these types of questions in a real interview after hearing the prompt to align the scope with the interviewer.

Best,

Alberto

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Check out my latest case based on a real MBB interview: Sierra Springs