Medium · Probability & Combinatorics · Numerical · ~6 min · #59
Waiting for Two Sixes in a Row
Real interview · Citadel Securities
You roll a fair die repeatedly. What is the expected number of rolls until you get two 6s in a row?
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Worked solution
Give the problem an honest attempt first, then reveal the full explanation.
Reported in interviews
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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