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Puzzles

 Collection of brain teasers I have found from various sources:

Weighing & Logical Deduction Puzzles

  1. Odd Coin Problem: You have 6 coins and a balance. One coin is either lighter or heavier than the rest. What is the minimum number of weighings needed to certainly find the odd coin?

  2. Finding the Heavier Ball: You have 27 balls, one of which is heavier. Using a balance scale, describe the steps to find the heavier ball in the minimum number of weighings.

  3. Defective Jar Puzzle: You have 10 jars filled with marbles. Each marble weighs 10 grams, except one jar, which contains defective marbles weighing 9 grams. Using one measurement on a modern digital scale, how do you find the jar with defective marbles?

Real-World Logic & Constraint-Based Puzzles

  1. Tunnel Escape Puzzle: Two people are walking through a railway tunnel. When they have crossed 2/3rd of the tunnel, they suddenly see a train approaching from behind. They immediately run in opposite directions, both at 10 km/h, and barely escape as the train passes. What is the speed of the train?

  2. Light Bulb & Switch Puzzle: You have three switches outside a closed room, each controlling one of three light bulbs inside. You may enter the room only once. How can you determine which switch corresponds to which bulb?

  3. Bridge Crossing at Night: Four people need to cross a bridge at night. The bridge can hold only two people at a time and requires a torch to cross. Each person walks at a different speed:

    • Person 1: 1 minute
    • Person 2: 2 minutes
    • Person 3: 5 minutes
    • Person 4: 10 minutes
      If two people cross together, they move at the speed of the slower person. What is the fastest time in which all four can cross?

  4. Age Riddle: A person's age in days is equal to his father's age in weeks, and his age in months is equal to his grandfather’s age in years. The sum of all three ages is 120 years. What are their ages?

  5. Burning Ropes: You have two ropes, each of which takes exactly one hour to burn completely. However, the ropes burn at an inconsistent rate. Using only a lighter, how can you measure exactly 45 minutes?

Coding & Algorithmic Thinking Puzzles

  1. Water Jug Puzzle: You have a 5-liter jug and a 3-liter jug, along with an unlimited water supply. There are no measurement markings on the jugs. How can you measure exactly 4 liters of water?

  2. Shooting, Drowning, and Hanging Riddle: A man shoots his wife, then holds her underwater for 5 minutes, and then hangs her. After 10 minutes, they both leave for dinner together. How is this possible?

  3. Doctor & Son Puzzle: A father and son are in a car accident. The father dies on the spot, and the son is rushed to the hospital. The doctor looks at the boy and says, "I can't operate on him—he's my son!" How is this possible?

  4. 100 Doors Puzzle: There are 100 closed doors in a row. A person walks through them 100 times following this pattern:

    • The first time, they toggle all doors (open all).
    • The second time, they toggle every second door (close doors 2, 4, 6, ...).
    • The third time, they toggle every third door (change the state of doors 3, 6, 9, ...).
    • This continues up to the 100th pass.
      After the 100th pass, which doors remain open?

  5. Prisoners and the Bulb Puzzle: There are 100 prisoners in separate, isolated cells. They have no way to communicate with each other, except that every day, one prisoner is randomly selected to enter a room with a single light bulb. They can either toggle the bulb (switch it on/off) or do nothing. The bulb is initially off. The warden tells them:

  • At any time, any prisoner can declare that all 100 prisoners have visited the bulb room at least once.
  • If the prisoner is correct, they are all set free; if wrong, they are executed.
  • The prisoners can discuss a strategy before the game starts.
    What is the best strategy for them to ensure their freedom?
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