Easy · Expected Value & Decision Theory · Numerical · ~3 min · #72

Counting Switches in Coin Flips

Real interview · J.P. Morgan

You flip a fair coin 100 times. A switch is a head immediately followed by a tail, or a tail immediately followed by a head. What is the expected number of switches?

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Worked solution

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

Reported in interviews

J.P. Morgan· Quantitative Research AnalystGlassdoor interview question
Asked for general N.

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

Linearity of expectationIndicator variables

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Quant ResearcherQuant TraderQuant Developer