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90 questions · full solutions

Brainteasers & Logic

The strategy-and-logic canon: weighings, crossings, hat protocols, and information arguments. The answer matters less than the invariant you spot on the way.

  1. 01Three Mislabeled JarsEasy
  2. 02Clock Hands at 3:15Easy
  3. 03Ants on a TriangleEasy
  4. 04Burning RopesMedium · Private
  5. 05The 100 LockersMedium · Private
  6. 06The Bridge at NightMedium · Private
  7. 0725 HorsesHard · Private
  8. 08The Two Egg ProblemHard · Private
  9. 09Twelve Balls, Three WeighingsHard · Private
  10. 10Five Pirates, 100 CoinsHard · Private
  11. 11Nine Balls, One HeavyEasy
  12. 12The Fork in the RoadEasy
  13. 13Three Switches, One BulbEasy
  14. 14The Round-Trip AverageEasy
  15. 15Two Trains and a FlyEasy
  16. 16Paying With a Gold BarEasy
  17. 17Splitting the CakeEasy
  18. 18Wine Into WaterMedium · Private
  19. 19The Monk on the MountainMedium · Private
  20. 20Twenty Heads in the DarkMedium · Private
  21. 21The Mutilated ChessboardMedium · Private
  22. 22Three and Five Make FourMedium · Private
  23. 23The Rolling CoinMedium · Private
  24. 24A Fair Toss From a Biased CoinMedium · Private
  25. 25Chameleon ConvergenceMedium · Private
  26. 26Spin or Don't SpinMedium · Private
  27. 27Nine Minutes From 4 and 7Medium · Private
  28. 28One Poisoned BottleMedium · Private
  29. 29One Hundred Hats in a LineMedium · Private
  30. 30Last One StandingMedium · Private
  31. 31Ant Across the CubeMedium · Private
  32. 32The Banana CaravanHard · Private
  33. 33Where to Stand in LineHard · Private
  34. 34The Handshake PartyHard · Private
  35. 35The Blue-Eyed IslandersHard · Private
  36. 36Strange Dice, Same SumsHard · Private
  37. 37The Light Switch ProtocolHard · Private
  38. 38Twenty Questions, One LieHard · Private
  39. 39The Last Chip PaysEasy
  40. 40Seven Cups, Two at a TimeEasy
  41. 41The Incompatible CargoEasy
  42. 42One True InscriptionMedium · Private
  43. 43Breaking the BarMedium · Private
  44. 44Nine and FifteenMedium · Private
  45. 45The Knight's Round TripMedium · Private
  46. 46Take 1, 2, or 4Medium · Private
  47. 47The Average Without the NumbersMedium · Private
  48. 48The Bent NeedleMedium · Private
  49. 49Twenty-One Matches in a RingMedium · Private
  50. 50Fifteen From Seven and ElevenMedium · Private
  51. 51Three Rounds, Shrinking PotMedium · Private
  52. 52Tiling the Courtyard with T'sMedium · Private
  53. 53Twelve Glasses, Five at a TimeMedium · Private
  54. 54Race to SixtyEasy
  55. 55The Torn LedgerEasy
  56. 56Three Drawers of ChipsMedium · Private
  57. 57Four Crates, All LiesMedium · Private
  58. 58The Poisoned CornerMedium · Private
  59. 59Ten Sides, Two SlotsMedium · Private
  60. 60One Question, Foreign WordsMedium · Private
  61. 61Four Pings in a Quarter ArcMedium · Private
  62. 62Coasters on a Round TableMedium · Private
  63. 63No Clean BreakMedium · Private
  64. 64Seven Badges, One Sure HitHard · Private
  65. 65Guess or Stay SilentHard · Private
  66. 66Forty Drawers, Twenty-Four PeeksHard · Private
  67. 67Last Match LosesHard · Private
  68. 68Two Stacks, Equal RaidsHard · Private
  69. 69Twelve Coins, Sealed OrdersHard · Private
  70. 70Two Bad Pallets, One ReadingHard · Private
  71. 71What Four Weighings Can ClearHard · Private
  72. 72Three Prototypes, Two Hundred NotchesHard · Private
  73. 73Eighteen Soldiers, Two Scouts, One CanoeHard · Private
  74. 74The Leak on the Endless PipelineHard · Private
  75. 75Eight Builds, One Lie, No AdaptingHard · Private
  76. 76Five Quotes, One ShotHard · Private
  77. 77Five Partners, Strict MajorityHard · Private
  78. 78The Compliance Email That Said Nothing NewHard · Private
  79. 79Peek, Then Switch?Hard · Private
  80. 80Two Broken TilesHard · Private
  81. 81Cable Trays in the Server HallHard · Private
  82. 82The Sliding Badge BoardHard · Private
  83. 83Unwind at the Same BellHard · Private
  84. 84The Hopping BugHard · Private
  85. 85Rank Five Ingots in SevenHard · Private
  86. 86Three Desks, No EnvyHard · Private
  87. 87Consecutive Badges, Alternating SilenceHard · Private
  88. 88Four Shards, One FrameHard · Private
  89. 89Size the FeedMedium · Private
  90. 90The Payoff Changes After You AnswerMedium · Private