91 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
  5. 05The 100 LockersMedium
  6. 06The Bridge at NightMedium
  7. 0725 HorsesHard
  8. 08The Two Egg ProblemHard
  9. 09Twelve Balls, Three WeighingsHard
  10. 10Five Pirates, 100 CoinsHard
  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
  23. 23The Rolling CoinMedium · Private
  24. 24A Fair Toss From a Biased CoinMedium
  25. 25Chameleon ConvergenceMedium · Private
  26. 26Spin or Don't SpinMedium · Private
  27. 27Nine Minutes From 4 and 7Medium · Private
  28. 28One Poisoned BottleMedium
  29. 29One Hundred Hats in a LineMedium
  30. 30Last One StandingMedium · Private
  31. 31Ant Across the CubeMedium
  32. 32The Banana CaravanHard
  33. 33Where to Stand in LineHard · Private
  34. 34The Handshake PartyHard · Private
  35. 35The Blue-Eyed IslandersHard
  36. 36Strange Dice, Same SumsHard
  37. 37The Light Switch ProtocolHard
  38. 38Twenty Questions, One LieHard · Private
  39. 39The Last One StandingMedium
  40. 40The Last Chip PaysEasy
  41. 41Seven Cups, Two at a TimeEasy
  42. 42The Incompatible CargoEasy
  43. 43One True InscriptionMedium · Private
  44. 44Breaking the BarMedium · Private
  45. 45Nine and FifteenMedium · Private
  46. 46The Knight's Round TripMedium
  47. 47Take 1, 2, or 4Medium · Private
  48. 48The Average Without the NumbersMedium · Private
  49. 49The Bent NeedleMedium · Private
  50. 50Twenty-One Matches in a RingMedium · Private
  51. 51Fifteen From Seven and ElevenMedium · Private
  52. 52Three Rounds, Shrinking PotMedium · Private
  53. 53Tiling the Courtyard with T'sMedium · Private
  54. 54Twelve Glasses, Five at a TimeMedium · Private
  55. 55Race to SixtyEasy
  56. 56The Torn LedgerEasy
  57. 57Three Drawers of ChipsMedium
  58. 58Four Crates, All LiesMedium
  59. 59The Poisoned CornerMedium · Private
  60. 60Ten Sides, Two SlotsMedium · Private
  61. 61One Question, Foreign WordsMedium
  62. 62Four Pings in a Quarter ArcMedium · Private
  63. 63Coasters on a Round TableMedium · Private
  64. 64No Clean BreakMedium · Private
  65. 65Seven Badges, One Sure HitHard · Private
  66. 66Guess or Stay SilentHard · Private
  67. 67Forty Drawers, Twenty-Four PeeksHard · Private
  68. 68Last Match LosesHard · Private
  69. 69Two Stacks, Equal RaidsHard · Private
  70. 70Twelve Coins, Sealed OrdersHard · Private
  71. 71Two Bad Pallets, One ReadingHard · Private
  72. 72What Four Weighings Can ClearHard · Private
  73. 73Three Prototypes, Two Hundred NotchesHard · Private
  74. 74Eighteen Soldiers, Two Scouts, One CanoeHard · Private
  75. 75The Leak on the Endless PipelineHard · Private
  76. 76Eight Builds, One Lie, No AdaptingHard · Private
  77. 77Five Quotes, One ShotHard · Private
  78. 78Five Partners, Strict MajorityHard · Private
  79. 79The Compliance Email That Said Nothing NewHard · Private
  80. 80Peek, Then Switch?Hard · Private
  81. 81Two Broken TilesHard · Private
  82. 82Cable Trays in the Server HallHard · Private
  83. 83The Sliding Badge BoardHard · Private
  84. 84Unwind at the Same BellHard · Private
  85. 85The Hopping BugHard · Private
  86. 86Rank Five Ingots in SevenHard · Private
  87. 87Three Desks, No EnvyHard · Private
  88. 88Consecutive Badges, Alternating SilenceHard · Private
  89. 89Four Shards, One FrameHard · Private
  90. 90Size the FeedMedium · Private
  91. 91The Payoff Changes After You AnswerMedium · Private