Logic Puzzles
Riddles that require logical thinking and problem-solving skills.
The Blind Man
A blind man walked into a restaurant and ordered a meal. When he was done, he asked for the bill, looked at it, and left a tip. How did he read the bill?
Answer: He didn't - the blind man never 'looked' at the bill
The Brother Question
Two fathers and two sons went fishing. They caught three fish which they divided equally, so each person got exactly one fish. How is this possible?
Answer: There were only three people: a grandfather, his son, and his grandson. The son is both a father and a son.
The Burning Rope Problem
You have two identical ropes that burn irregularly. Each rope takes exactly 60 minutes to burn completely. How can you measure 45 minutes?
Answer: Light both ends of one rope and one end of the second rope. When the first rope is completely burned (30 minutes), light the other end of the second rope. When the second rope is completely burned, 45 minutes will have passed.
The Prisoner’s Hat Puzzle
Three prisoners are lined up single file, all facing forward. A wall separates the first prisoner from the other two. They are told that they will have either a black or a white hat placed on their head. Each prisoner can see the hats of the people in front of them, but not their own hat or the hats behind them. The third prisoner (at the back) can see the hats of the two people in front. The second person can see only the hat of the first person. The first person can’t see any hats. They are asked to identify the color of their own hat. If they guess wrong, they will be executed. If they refuse to guess, they will be imprisoned for life. How can they maximize the number of people who correctly identify their hat color?
Answer: The third prisoner counts the number of black hats they see. If it's odd, they say 'black'; if it's even, they say 'white'. The second prisoner then knows their hat color based on what the third prisoner said and what they see. The first prisoner has a 50% chance.
The Poisoned Wine
A king has 1000 bottles of wine. A neighboring queen plots to kill him and has poisoned one of the bottles. The poison is so potent that even if diluted by a million, it would still kill the king. The poison takes 24 hours to take effect. The king has 10 servants. How can he determine which bottle is poisoned in just one day?
Answer: Number the bottles 1 to 1000 in binary. Have each servant drink from all bottles where their position bit is 1. After 24 hours, the pattern of dead servants will give the binary representation of the poisoned bottle.
The Prisoner Dilemma
You’re in a prison with 2 doors. One leads to freedom, the other to execution. There are 2 guards, one by each door. One guard always tells the truth, the other always lies. You don’t know which is which. You can ask one question to one guard to determine the door to freedom. What do you ask?
Answer: 'If I asked the other guard which door leads to freedom, what would he say?' Then choose the opposite door.
The Rope Bridge Puzzle
Four people need to cross a rickety bridge at night. They have one flashlight and the bridge is too dangerous to cross without it. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins, and 10 mins. What is the shortest time needed for all four to cross the bridge?
Answer: 17 minutes (1 & 2 cross, 1 returns, 7 & 10 cross, 2 returns, 1 & 2 cross)
The Elevator Riddle
A man lives on the 10th floor of a building. Every morning he takes the elevator down to the ground floor to go to work. When he returns in the evening, he takes the elevator to the 7th floor and walks up the stairs to his apartment on the 10th floor. However, on rainy days, he takes the elevator all the way to the 10th floor. Why?
Answer: The man is a dwarf/short person who can't reach the 10th floor button, but on rainy days he has an umbrella to press it
The Lake Crossing Puzzle
There are 3 missionaries and 3 cannibals who need to cross a river. The boat can only hold 2 people at a time. If the cannibals ever outnumber the missionaries on either side of the river, the missionaries will be eaten. How can they all cross safely?
Answer: Two cannibals cross to the right bank (Left: 3M, 1C; Right: 0M, 2C) One cannibal returns to the left bank (Left: 3M, 2C; Right: 0M, 1C) Two cannibals cross to the right bank (Left: 3M, 0C; Right: 0M, 3C) One cannibal returns to the left bank (Left: 3M, 1C; Right: 0M, 2C) Two missionaries cross to the right bank (Left: 1M, 1C; Right: 2M, 2C) One missionary and one cannibal return to the left bank (Left: 2M, 2C; Right: 1M, 1C) Two missionaries cross to the right bank (Left: 0M, 2C; Right: 3M, 1C) One cannibal returns to the left bank (Left: 0M, 3C; Right: 3M, 0C) Two cannibals cross to the right bank (Left: 0M, 1C; Right: 3M, 2C) One cannibal returns to the left bank (Left: 0M, 2C; Right: 3M, 1C) Two cannibals cross to the right bank (Left: 0M, 0C; Right: 3M, 3C) All missionaries and cannibals have safely crossed to the right bank, and at no point did the cannibals outnumber the missionaries on either bank (except when there were 0 missionaries on that bank).
The Water Jug Problem
You have a 3-gallon jug and a 5-gallon jug. How can you measure exactly 4 gallons of water?
Answer: Fill the 5-gallon jug, pour into 3-gallon jug until full (leaving 2 gallons in 5-gallon jug). Empty the 3-gallon jug, pour the 2 gallons into it. Fill the 5-gallon jug again and pour 1 gallon into the 3-gallon jug (until it's full). This leaves exactly 4 gallons in the 5-gallon jug.