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Questions tagged [logical-deduction]

A puzzle that requires formal logical deduction to arrive at the solution. This suggests more than merely reasoning through clues to find an answer (you might want [situation] for that).

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37
votes
7answers
8k views

Guessing hat colors. 4 prisoners

This is a variant about guessing hat colors. You may want to try Hats and alien or Guessing hat colors or Four prisoners wearing black and white hats. Statement 4 prisoners are to be executed. ...
37
votes
3answers
5k views

The coolest checkerboard magic trick

In the small town of Terni (Italy), there's a couple of young friends named Marco and Leonardo, who like to perform magic tricks to a restricted audience of common friends and relatives. They like to ...
37
votes
5answers
12k views

Duel between the knight and the dragon

A dragon and a knight live on an island where the only sources of fresh water are a pond (containing ordinary water) and six wells, numbered 1 to 6. Each well's water is completely indistinguishable ...
36
votes
2answers
4k views

What is the result of the equation?

Considering from $a$ until $z$: $x = (x -a) \cdot (x -b) \cdot (x -c) \dots (x - z)$ What I want is the value of $x$ It´s something easy, but I could not find this here and I think it´s a nice ...
36
votes
12answers
5k views

Find the liar in the library

I've just taken a CS exam, which had the following question 6 persons entered a library the day a book got stolen. Each of them entered the library once and only once, and stayed there for some ...
36
votes
4answers
5k views

Who poisoned Hugh G. Pokitz? (a murder mystery logic puzzle)

Preface After your last mishap with a heap full of rabbits, you got sick of being lied to. Isn't life just so much easier when you can conclusively tell if someone's been telling the truth or not? ...
36
votes
5answers
10k views

Sabotage at Sea - Cursed Cruise liner?

(Warning: Heavy reading incoming. I suggest writing stuff down...) It's now the year 2150, and you're one of the few people who didn't leave earth twenty years ago. Why? A deadly disease overtook the ...
36
votes
1answer
2k views

Introducing: Number Slope™

Number Slope™1 is an original2 grid deduction puzzle similar to Sudoku. It is solved on an n-by-n grid, tiled with n n-ominoes. The rules are: Each number from 1 to n must occur once and only once in ...
36
votes
2answers
6k views

Escape the dungeon, by deciphering the codes and navigating the rooms to find a way out

You have been thrown into a dungeon with many rooms; you don't know who did this or why. You start in room 37. You might be thinking "Why 37?", so I will explain this: "Why not?" You need to find ...
35
votes
12answers
9k views

Batman vs 4 villains

One day after having his morning Bat-coffee, Batman received the following message on his Bat-computer: Hello there Dark Knight. I, the Riddler, along with Joker, Two-Face, and Penguin, are ...
35
votes
10answers
5k views

Knights and Knaves: What does C say?

You are on a fictional island with two types of people: knights who always tell the truth, and knaves, who always lie. Three of the inhabitants - A, B, and C are standing in the garden. A says, "B and ...
35
votes
13answers
7k views

6 prisoners, 2 colors, one mute

Just like the classic 4 prisoners hats riddle, here we have 6 prisoners buried to their necks in the ground. They can only look straight ahead so that A only sees B, C, D, E while B sees C, D, E, and ...
35
votes
8answers
7k views

Who committed the crime?

A crime has been carried out by one person, there are 5 suspects. Each suspect is asked under polygraph who they think committed the crime. Their answers are as follows: Terry : It wasn't Carl, It ...
35
votes
1answer
1k views

“I can use computers, grandson!”

My grandmother loves matchstick puzzles. Every time I visit, she shows me a brand new set for me to solve. I visited her recently for her birthday, and I was excited to receive a puzzle in exchange ...
34
votes
16answers
5k views

XKCD inspired logic puzzle

I came up with a logic puzzle inspired by this xkcd comic. I don't yet have an answer, and I am wondering if anyone can come up with one. You are in a room with three doors. Two of the doors lead ...
34
votes
3answers
5k views

Do better than chance

You and an opponent are playing the following game: Your opponent picks two different real numbers, however he wants to, and writes them on two cards, which he presents to you face down You choose ...
34
votes
6answers
1k views

Sudoku embedded in a Kakuro

In addition to standard Kakuro rules, the grey shaded cells must form a valid Sudoku solution. One sum in this puzzle has been corrected from 28 to 23. Fortunately, it was in a location that nobody ...
33
votes
15answers
39k views

Mysterious Murder Mystery [closed]

Last year, December 21, 2013, you were awarded the "#1 Detective in the World" for solving more than 50 murders. Your boss gave you a promotion, contingent upon your moving to Chicago. You agreed and ...
33
votes
12answers
8k views

The total cost of salaries in a company

Story Version (scroll for TL;DR) It's the end of the first quarter of 2017. This means it's the end of the 2016 fiscal year (at least in my country it is). It's time to draw the line. So the CEO did ...
33
votes
9answers
4k views

Winning Strategy for the Magician and his Apprentice

There are $13$ upside-down opaque cups and $2$ balls, a magician and his apprentice and yourself. You decide under which cups to put the balls, and the objective of the magician is to find the two ...
33
votes
9answers
6k views

Find the composite poison in the wine bottles

You are the ruler of a medieval kingdom who loves throwing parties. The courtier who tried to poison one of your wine bottles last time was furious to learn that you managed to identify which bottle ...
33
votes
3answers
4k views

What color is the drummer's hair?

Three friends - a cricketer named White, a footballer named Black and a drummer named Redhead met in a cafeteria. "It is remarkable that one of us has white hair, another one has black hair and the ...
33
votes
3answers
5k views

Pirates and gold coins

A group of N pirates has come by a chest containing 200 gold coins. Their rules require that the coins be distributed by the following approach. The pirates are ranked from fiercest to meekest and ...
33
votes
2answers
748 views

lolcatz can haz ur infinit cheeseboard

This is a problem on an infinite chessboard with pieces called lolcatz. They can move like queens or knightriders, but have a strange disadvantage... ... you've never heard of knightriders? Well, ...
33
votes
2answers
969 views

Mystery of the One Word Wordsearch

How have you been? Please, come in, have a seat. Would you like a drink? Wine? Soda? No? Yes, I have orange juice... Ok, enjoy! Anyway, I know how you love wordsearches, so I made one exclusively for ...
33
votes
3answers
5k views

Mate in one with NO PIECES?

The following pictures depict chess positions one move before white checkmated black: The problem is, I seemed to have completely lost which pieces each letter/number represents. All I know is that ...
32
votes
4answers
5k views

Honest Monk Puzzle

Suppose you are on an island with 32 monks and you are told that one of the monks is always honest. Every day the monks say whether it is going to rain. You are required to say whether it is going to ...
32
votes
1answer
2k views

The mystery flag

Part two - The mystery flag - twisted Based on these flags: (Count the sun in the Argentina flag as yellow) Which country does this flag belong to? Please explain why EDIT: I added a black ...
31
votes
8answers
10k views

How many cars does the millionaire have?

I had a logical riddle on a programming interview test which was something like this: In his garage a millionaire had cars, of which only 2 were not white, only 2 were not green and only 2 were not ...
31
votes
3answers
4k views

The ping pong puzzle

Three friends ($A$, $B$ and $C$) are playing ping pong. They play the usual way: two play at a time, the winner stays on, and the loser waits his/her turn again. At the end of the day, they summarise ...
31
votes
12answers
6k views

Knights, Knaves and Normals

In the kingdom of Boolistan, every inhabitant is either a Knight, Knave or Normal. Knights can only make true statements, Knaves can only lie, and Normals must either tell the truth or lie. Warmup: ...
31
votes
10answers
6k views

100 Prisoners and a clock

There are 100 Prisoners who are given a chance at freedom. The prisoners are randomly picked to visit a room where there is only a nonfunctional wall clock with a ...
30
votes
12answers
9k views

Correct way to add 22 to 4 to get given value

I was asked this logic puzzle in an interview. Tell me a correct way by which adding 22 to 4 will give 2. I asked interviewer what is the base of those numbers. He told that those are decimal ...
30
votes
6answers
18k views

Nine Bottles of Wine, One Poisoned, Two Mice, Two Tests

Came across this puzzle and couldn't think of a solution: You are given nine bottles of wine out of which one is poisoned. Given two mice (that will die when taken a sip of poisoned wine), how ...
30
votes
4answers
6k views

Half filled water bottle

If you are given a marker and a transparent water bottle partially filled with water, can you tell if the bottle is half filled or not?
30
votes
6answers
5k views

Who's been stealing Jenny's milk?

Jenny works in an office with four other colleagues and someone has been using her milk. She finds this very annoying as she likes her mid-afternoon tea and she simply can't enjoy the second half of ...
30
votes
7answers
5k views

Dinosaur egg drop

Archeologists discover two dinosaur eggs, and you are given the chance to test the durability of these eggs (bad move on their part). Suppose that these eggs will absorb a specific amount of force ...
30
votes
2answers
2k views

Question 16 Ripple Effect

This is a Ripple Effect puzzle. The rules of this puzzle are: Fill in each region with numbers from 1 to n, where n is the region size. For instance, the top left region must have the numbers 1 to 5 ...
30
votes
7answers
4k views

How long has the warden been the warden?

Summary: It is your first day in prison serving a literal life-sentence under the supervision of the warden. You are to be kept in solitary confinement with no additional information for the ...
30
votes
5answers
6k views

Two spies throwing stones into a river

There is a puzzle about two spies: Two spies must pass each other two secret numbers (one number per spy), unnoticed by their enemies. They have agreed on a method for doing this using only 26 ...
29
votes
8answers
5k views

A long, lonely drive

It's early in the wee hours of the morning when a lonely farmer loads up his truck with his goods to sell. He lives far from the city and if he wants to ensure a good spot at the bazaar, he will need ...
29
votes
7answers
7k views

Guy who understands if a diamond is genuine by just looking at it

There is a guy who can determine if a diamond is genuine or fake by just looking at it. His name is Richard. One day, he is summoned by the leader of a gang of thieves. It turns out that thieves have ...
29
votes
5answers
4k views

Color elimination puzzle

You have a 5x5 board with on each cell a light that can be red, green or blue. Next to each row and each column there is a button that when pressed makes all red lights in that row/column green, all ...
29
votes
1answer
3k views

Murdered down the Rabbit Hole (a long logic puzzle)

Backstory Disaster has struck for Leporidae throughout the fantasy realm -- a mass carrot famine has wreaked havoc upon all existences but our own. Because of this, 25 of the leading fantasy bunnies, ...
28
votes
13answers
93k views

Three Doors, One Truth-Teller, One Question

You are in a prison. There are 3 doors, and a ghost standing in the middle of the prison. He says one of the doors is the way to freedom while the other two lead to death. You get to ask only one ...
28
votes
3answers
3k views

12 Birds in the petshop

You decided to buy 2 birds from a pet shop and went to a pet shop for it. There are 12 budgerigars at the pet shop and you want a male and a female for your home. You cannot distinguish the gender of ...
28
votes
2answers
1k views

Reverse Puzzling 2

This puzzle was inspired by Dr Xorile's first reverse puzzle! Kudos to him for coming up with such a fun format. Intrigued, I gave George yet another famous puzzle to ponder over. Once again, I was ...
28
votes
3answers
3k views

Coloring a 4 by 4 grid

I made this simple puzzle a couple of years ago. There are three colors which appear in every row and every column. Moreover, every color appears 5 or 6 times in total. Find the color of the square ...
27
votes
6answers
5k views

Out of these 20 Doors, Which One Leads You to the Jackpot?

Congratulations for achieving this final stage of "Millionaire Super Logic" show! Once again, my name is Athin, and I'll be your host to help you win the jackpot of one, milLION, DOLLARS! *audience ...
27
votes
3answers
4k views

Differentiate between the numbers from 1 to 5 with one single yes/no question

This question requires a bit of logical distortion to get the information required to answer the question. However, I've never seen it asked before what would happen if more modes of question ...