Easy · Probability & Combinatorics · Numerical · ~5 min · #115

Waiting for HHT

Real interview · Jane Street, Deutsche Bank

You flip a fair coin repeatedly. What is the expected number of flips until the pattern HHT first appears?

Hints · 0 of 2 shown

Worked solution

Give the problem an honest attempt first, then reveal the full explanation.

Reported in interviews

Jane StreetGlassdoor interview question
Deutsche Bank· Quantitative AnalystGlassdoor interview reviews

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.

Concepts

First-step analysisPattern waiting times

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