If it is a Market Size case, and the context about "guesstimates" (i think it is). First of all, you need to reconcile your answer (est. earnings) to baseline from which it was derived. Example: what % does estimated earnings take from total earnings? Secondly, - as i understand, you're always expected to provide recommendations how to make your estimation more relevant. I don't understand how it is possible to start calculate historical ratios on the case interview.
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