← Interview questions (20) 100 questions Choose a title to open its card
R001 Chocolate without shortcutsCounting Easy ↗ R002 Socks in a dark drawerPigeonhole principle Easy ↗ R004 Nine tokens, one heavyInformation Easy ↗ R005 A broken chessboardInvariants Easy ↗ R006 The lockers that stay openNumber theory Easy ↗ R008 Zeroes at the endNumber theory Easy ↗ R010 An odd ring of gearsInvariants Easy ↗ R011 A contradictory gridParity Easy ↗ R012 Six yes-or-no questionsInformation Easy ↗ R013 Replace two numbersInvariants Easy ↗ R014 Water exchanged between two cupsConservation Easy ↗ R015 An impossible handshake reportGraph reasoning Easy ↗ R016 A shared last digitPigeonhole principle Easy ↗ R018 Take the last counterStrategy Easy ↗ R061 A tournament with byesCounting Easy ↗ R062 Merge two ordered queuesCounting Easy ↗ R063 Every container is mislabelledDeduction Easy ↗ R064 Audit an if-then ruleLogic Easy ↗ R065 Two prerequisites and one releaseOptimization Easy ↗ R066 Four weights for every integer loadInformation Easy ↗ R067 A panel with pair switchesParity Easy ↗ R068 The score from splitting a pileInvariants Easy ↗ R069 Routes around a blocked junctionCounting Easy ↗ R070 Round each invoice or round the total?Bounds Easy ↗ R071 Where should the depot go?Optimization Easy ↗ R072 Keep subtracting the smaller numberNumber theory Easy ↗ R073 How many meeting rooms?Scheduling Easy ↗ R074 Remove one or fourStrategy Easy ↗ R075 A checksum and one missing bitInformation Easy ↗ R076 Reverse a queue by neighbouring swapsOptimization Easy ↗ R021 Crossing with a single torchOptimization Medium ↗ R022 Two uneven-burning cordsMeasurement Medium ↗ R023 Split the face-up coins blindfoldedInvariants Medium ↗ R024 Four litres, no markingsMeasurement Medium ↗ R025 One faulty batch, one digital weighingInformation Medium ↗ R026 Testing a thousand faulty chipsInformation Medium 🔒 R027 Two eggs and 36 floorsOptimization Medium 🔒 R028 Three piles, one winning moveStrategy Medium 🔒 R029 The missing square and the L tilesInduction Medium 🔒 R030 Two numbers, one dividing the otherPigeonhole principle Medium 🔒 R031 Two equal handshake countsGraph reasoning Medium 🔒 R032 A truth table in disguiseLogic Medium 🔒 R033 A number made only of ones and zeroesNumber theory Medium 🔒 R034 Two-colour token replacementInvariants Medium 🔒 R035 A rectangle's diagonal routeGeometry Medium 🔒 R036 Match the smallest with the largestOptimization Medium 🔒 R038 Rectangles hidden in a gridCounting Medium 🔒 R039 The poisoned last counterStrategy Medium 🔒 R077 Colliding robots on a railSymmetry Medium 🔒 R078 Several faulty batches, one measurementInformation Medium 🔒 R079 Tests that remember when they failedInformation Medium 🔒 R080 The host's spouseGraph reasoning Medium 🔒 R081 Find the majority without storing the listInvariants Medium 🔒 R082 Find the person everyone knowsDeduction Medium 🔒 R083 Find the three fastest without a stopwatchOrder reasoning Medium 🔒 R084 Choose the most appointmentsScheduling Medium 🔒 R085 Minimize total waiting for completionOptimization Medium 🔒 R086 Pair the numbers to maximize valueOptimization Medium 🔒 R087 The centre of a magic squareConstraints Medium 🔒 R088 An even total that still cannot happenGraph reasoning Medium 🔒 R089 A sparse rulerMeasurement Medium 🔒 R090 Move a tower and prove the costInduction Medium 🔒 R091 Recover a deleted digitNumber theory Medium 🔒 R092 Choose the row before or after the columnBounds Medium 🔒 R093 Split consecutive labels equallyConstruction Medium 🔒 R041 Hat colours as a communication channelInformation · MULTI-PART Hard ↗ R042 Sorting with only three-cyclesInvariants · MULTI-PART Hard ↗ R043 A balance plan fixed in advanceInformation Hard ↗ R044 The silent badgesLogic Hard 🔒 R045 A meeting room with one persistent bitInformation Hard 🔒 R046 Two eggs, then threeOptimization · MULTI-PART Hard 🔒 R047 Three mutual friends or strangersGraph reasoning Hard 🔒 R048 A divisible consecutive blockPigeonhole principle Hard 🔒 R049 Only ones, no zeroesNumber theory Hard 🔒 R050 Points in a small squareGeometry Hard 🔒 R051 Compare to find both extremesOptimization · MULTI-PART Hard 🔒 R052 A row of coins against an opponentStrategy Hard 🔒 R053 Find a missing label with tiny memoryInformation Hard 🔒 R054 A tournament's indirect winnerLogic Hard 🔒 R055 A board that cannot turn all whiteInvariants Hard 🔒 R056 Find a faulty reading with one lieInformation Hard 🔒 R057 A product concealed in a sumNumber theory · MULTI-PART Hard 🔒 R058 A route using every connection onceGraph reasoning Hard 🔒 R059 A symmetric placement gameStrategy Hard 🔒 R060 The second-largest needs a witness listOptimization Hard 🔒 R094 Supplies consumed during transportOptimization · MULTI-PART Hard 🔒 R095 Can three colours become one?Invariants · MULTI-PART Hard 🔒 R096 The council's allocation voteBackward induction · MULTI-PART Hard 🔒 R097 Assign projects without a dead endMatching · MULTI-PART Hard 🔒 R098 A spreading fault on a gridInvariants · MULTI-PART Hard 🔒 R099 Balance loads without cyclingTermination · MULTI-PART Hard 🔒 R100 The switch starts in an unknown stateInformation · MULTI-PART Hard 🔒 R101 One flip communicates a secret squareInformation · MULTI-PART Hard 🔒 R102 Four visible cards identify the fifthInformation · MULTI-PART Hard 🔒 R103 Connect red and blue without crossingsGeometry · MULTI-PART Hard 🔒 R104 Find a target in either directionAdversarial search · MULTI-PART Hard 🔒 R105 A long rising or falling subsequencePigeonhole principle · MULTI-PART Hard 🔒 R106 Twelve suspect coins without a reference coinInformation · MULTI-PART Hard 🔒 R107 A ranking without a transitive winnerOrder reasoning · MULTI-PART Hard 🔒 R108 Complete a circular route with no initial fuelInvariants · MULTI-PART Hard 🔒 About this collection & research sources Original practice wording and extensions informed by your Quant Trading Guide, quant book and interview question bank, plus the public sources below. Selected for mathematical reasoning relevant to quant preparation, not claimed to be questions asked by a particular firm. Random sampling, expected values and probability calculations belong in Probability.
NRICH: Crossing the Bridge · Berkeley: Hat puzzles · Jane Street: Interview problem solving · Toronto: Invariants · Boyer–Moore: Majority cancellation · Oxford Maths Club: Invariants
Difficulty is an editorial guide. These are practice problems, not claims about questions asked by a particular firm. Read the stated assumptions carefully; a different model can change the answer.