Hard · Expected Value & Decision Theory · Numerical · ~15 min · #60
Seeing Every Face Twice
Real interview · Citadel Securities
You roll a fair die until every face has appeared at least twice. What is the expected number of rolls? Give a numerical answer.
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Worked solution
Give the problem an honest attempt first, then reveal the full explanation.
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Candidate-reported and unverified; the firm may phrase it differently or no longer ask it. The wording here is our own paraphrase, and the solution was written and checked independently of any answers posted at the source.
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