Easy · Probability & Combinatorics · Numerical · ~4 min · #94
One Double-Headed Coin in a Thousand
Real interview · Goldman Sachs, Squarepoint Capital +1
A bag has 999 fair coins and 1 double-headed coin. You draw one at random and flip it 10 times, getting 10 heads. What is the probability that you hold the double-headed coin?
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Worked solution
Give the problem an honest attempt first, then reveal the full explanation.
Reported in interviews
- Goldman SachsGlassdoor interview question
- Squarepoint CapitalGlassdoor interview question
- Renaissance TechnologiesGlassdoor interview question
Reported without numbers, as finding a two-headed coin in a pile.
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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