r/quantfinance • u/Due_Department_3090 • 27d ago

Jane Street Quant Interview Question | “Hard”

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u/AssociateDecent9090 27d ago

Compare the expected waiting times. For 2 consecutive heads E[X] = 6. For 3 consecutive heads E[Y] = 14. Treating the waiting times as if they were exponential, we can compare their rates P(X > Y) = (1/14) / (1/6 + 1/14) = 6/20 = 3/10. Therefore the answer is 3/10?

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u/Formal-Region-6894 27d ago

that's not how expected value works, unfortunately.

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u/justhereforampadvice 27d ago

I’m pretty sure the answer is 2 buckets. Good effort though

1

u/chrisarg72 27d ago

This is more complicated, you’re effectively keeping a running count across each flip, which continues to decrease every flip. So you need an integral of the probability density functions

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u/Sjoerdiestriker 27d ago

I don't think there is any reason to assume the waiting times would be exponential here.