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Previous problems of the day

The last 44 daily picks, newest first. Days follow UTC. Each problem keeps the same number everywhere on KnowQuant, so a question you saw on the homepage is easy to find again.

Today, Sat, Oct 31, 2026

#013The larger of two diceStandard

Earlier days

  1. #012Roll a die, take it or rerollExpected valueWarm-up
  2. #011Hats returned at randomProbabilityStandard
  3. #010The test that is usually wrongProbabilityStandard
  4. #009The two envelopesProbabilityStandard
  5. #008Returning to the originProbabilityHard
  6. #007Waiting for two heads in a rowProbabilityHard
  7. #006Shared birthdaysProbabilityStandard
  8. #005Breaking a stick into a triangleProbabilityHard
  9. #004First head winsProbabilityStandard
  10. #003The three doorsProbabilityWarm-up
  11. #002Two children, one is a boyProbabilityWarm-up
  12. #001The final rollProbabilityHard
  13. #045The bug that only shows up in productionCodingWarm-up
  14. #044Price an option by simulationCodingStandard
  15. #043Why the loop order mattersCodingStandard
  16. #042Sample from a stream of unknown lengthCodingStandard
  17. #041Design a limit order bookCodingHard
  18. #040Rolling median of a streamCodingHard
  19. #039Maximum drawdown in one passCodingWarm-up
  20. #038Why n minus oneStatisticsWarm-up
  21. #037The trend that reversesStatisticsStandard
  22. #036Is this series stationary?StatisticsStandard
  23. #035The backtest that will not surviveStatisticsStandard
  24. #034How long to trust a Sharpe ratioStatisticsHard
  25. #033Why the top performers disappointStatisticsWarm-up
  26. #032The third correlationStatisticsStandard
  27. #031How much do you bet?Market makingStandard
  28. #030Price something you cannot look upMarket makingHard
  29. #029How wide should the spread be?Market makingHard
  30. #028You are long and you do not want to beMarket makingStandard
  31. #027The informed counterpartyMarket makingStandard
  32. #026Make a market on two diceMarket makingWarm-up
  33. #025Ants on a poleBrainteasersStandard
  34. #024Twelve balls, three weighingsBrainteasersHard
  35. #023Estimate the piano tunersBrainteasersStandard
  36. #022The blue-eyed islandersBrainteasersHard
  37. #021One thousand bottles, one poisonedBrainteasersStandard
  38. #020One hundred prisoners and one hundred boxesBrainteasersHard
  39. #019Two ropes, forty five minutesBrainteasersWarm-up
  40. #018Uniforms until the sum passes oneExpected valueHard
  41. #017Gambler's ruinExpected valueStandard
  42. #016When to stop lookingExpected valueHard
  43. #015Collecting the full setExpected valueStandard
  44. #014Cards until the first aceExpected valueHard

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