I think the "largest retail customer" is the largest shop that sells our products. That is, the shop from which we gain most of our sales revenue.
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The case study is the most important element of the case interview, which you'll have to nail in order to get into strategic consulting. Here you can learn the specific skills and concepts necessary to solve them.
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In order to repeatedly demonstrate prerequisite skills under the pressure of a real case interview, you need to learn the basics and practice cases.