library / Puzzle Lab / Probability Puzzles / st-petersburg-startupreviewed 2026-08-12

Puzzle Lab / Probability Puzzles

The Infinite Startup Payout

A probability puzzle about expected value, real-world constraints, and why math without context lies politely.

Room context

Puzzle

Challenge first.

A fair coin is tossed until the first heads. If heads appears on toss n, you win 2^n rupees. The expected value is mathematically infinite. Should you pay any finite amount to play?

Hints
  1. The expected value calculation is not the whole decision.
  2. Ask who can actually pay the largest outcomes.
  3. Utility is not linear in money.
Solution

The formal expected value diverges, but real decisions have bankroll limits, payout limits, risk preferences, and diminishing utility. The puzzle separates algebra from decision-making.

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