St Petersburg paradox
Definition · Level 11 · Interview practice
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A game paying $2n if the first tail comes on flip n: each n adds $1 of expected value, so the sum is infinite — yet against a payer who holds ≈ $1M (≈ 220) it is worth only about $21.
Example
“Infinite on paper; capped at your bankroll, about twenty dollars.”
Where Tradecraft teaches it
Level 11 · Interview practice, in the lesson “Probability & expected value classics”: The puzzles interviewers often recycle, and the five tools that crack them.
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