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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21
votes
8answers
3k views

Robots in a spaceship

Xerxes has 6, 7 and 8-arm robots in his spaceship. Unfortunately, the 7-arm robots are misprogrammed and everything they say is a lie. The other robots are fine, they always speak the truth. Once ...
21
votes
9answers
4k views

Which box is safe to open?

One day, you enter your house...and find it has been burglarized. On the table, a note says, "Thank you for your valuables. In return, I left a gift in the basement." Quite confused, you head down to ...
21
votes
9answers
3k views

House Number Mastermind

I was having a house-leaving party at my old house, and challenged four of my friends to guess the number of the new address, which had three digits. The responses I got were 280, 376, 304, and 370....
21
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5answers
3k views

Knights and Knaves : Liar , Liar - How many are you?

There are 2014 persons in a row Each of them is either a Knave (who always lies) or a Knight (who always tells the truth). Each person says 'There are more knaves to my left than knights to my ...
21
votes
8answers
11k views

Which one is the true statement? [closed]

All five statements below are true. None of the four statements below are true. Both of the statements above are true. Exactly one of the three statements above is true. None of the four statements ...
21
votes
3answers
3k views

Ten Tricky Mathematicians

Somewhere on the Hyperbolic Plane, you encounter ten mathematicians. You know that some of them always tell the truth while the rest always lie. Each of them knows which of the others tell the truth ...
21
votes
3answers
3k views

My wall clock is not working

When I came back from my vacation I saw my wall clock has fallen down and not working. The clock was on the floor and all its numbers were detached from their positions. Can you help me to find out at ...
21
votes
1answer
2k views

How do I tell the children's ages?

My friend once told me a logic puzzle along the lines of: A student asks a teacher what the ages of the teacher's three kids are. The teacher said, "the product of their ages is 72, and the sum of ...
21
votes
3answers
3k views

Winning an unfair game

I came across the following interesting problem today: A game consists of a sequence of plays; on each play either you or your opponent scores a point, you with probability $p < \frac{1}{2}$, he ...
21
votes
2answers
471 views

Statue View: Tetrominoes

This is a Statue View puzzle, an original invention combining two logic puzzle genres: Statue Park and Canal View. Rules of Statue View: Shade some cells of the grid to form the given set of pieces. ...
21
votes
1answer
1k views

My Grandfather's Final Message

As my grandfather reached the end of his long and satisfying life, I sat by his bedside and asked him what advice he could give me to help me in my own. Unable to speak, he pointed to his bedside ...
21
votes
1answer
3k views

Logic puzzle: Driving around the United States

For Super Bowl Sunday, here's a super-sized logic puzzle! This was a practice puzzle for a hunt I participated in a few years ago. There are ten delivery drivers driving around the United States of ...
20
votes
8answers
10k views

Logic puzzle - 3 suspects

An inspector knows that exactly one of 3 suspects committed a crime, and interviews them to find out which. Each person lies one time, and tells the truth the other time. A says: I did not do it. B ...
20
votes
3answers
2k views

Can you put L trominos to fill the figure?

In the above picture, there are 24 squares. Can you only use L trominos to fill the figure? If yes, give an example. Otherwise, please explain why. An L tromino is like this:
20
votes
5answers
5k views

Can you open the door or die? v2

This is the follow up question to my previous question You wake up and find yourself in a very strange room with a single door. You find a note on the ground surrounded by lots of cards: ...
20
votes
3answers
6k views

The lake monster

You are on a rowboat in the middle of a large, perfectly circular lake. On the perimeter of the lake is a monster who wants to eat you, but fortunately, he can't swim. He can run (along the perimeter)...
20
votes
5answers
2k views

Knights and knaves in Landilandia

You're traveling on a hiking trip in the land of Landilandia when you encounter a forest. The notes in your guidebook tell you that there's a safe route here leading into the woods, but when you ...
20
votes
3answers
1k views

The mysterious card in the middle

The referee tells Lefty, Righty, and Middly: "I have picked three positive integers $L,M,R$ with $L<M<R$ and $L+M+R=13$, and I have written these numbers onto the three cards lying face down on ...
20
votes
7answers
3k views

Ali Baba and the 10 thieves

EDIT: This puzzle originates from the "International Mathematics Tournament of the Towns", and was published in the book "S.M.A.R.T circle overview" by Professor Andy Liu. The author has (albeit a ...
20
votes
2answers
10k views

Coin weighing problem

You are given ten stacks of golden coins, each stack consisting of ten coins and a digital scale with arbitrary precision. You know that all stacks of coins are made from gold, weighing 10 grams per ...
20
votes
2answers
2k views

Emeralds bet - A round world

It was the 15 October 1502, just a few hours before midnight in an old tavern at the docks. Emerald raised two fingers, after repeatedly looking at his three cards, before finally answering the ...
20
votes
3answers
3k views

Sum of secret numbers is 101

Alice, Bob, and Charlie were given three numbers, respectively. They were said that all the three numbers were positive integers and the sum was 101. Alice: I know we have different numbers. Bob: ...
20
votes
1answer
2k views

What will the Frenchman say?

An Englishman, a Frenchman and a German are speaking. First the Englishman says : "M". Then the Frenchman says : "M" as well. And the German also says : "M". They all agree and decide to ...
20
votes
3answers
1k views

Edward and the seven imps' Game

Excerpt of “Manipulating the Fabric of Space” “Miscasting translocation spells can have many nasty consequences. The caster’s body could be warped or torn apart, his soul could be separated ...
20
votes
1answer
981 views

Isometric Heyacrazy - Now In 3D!

This is a Heyacrazy puzzle on an isometric grid. Rules of Heyacrazy: Shade some cells of the grid. Shaded cells cannot be orthogonally adjacent; unshaded cells must be orthogonally connected. ...
20
votes
1answer
2k views

The Illuminati and the Knights Templar

Some people in the American Senate are working for secret organizations. Some work for the Illuminati, some work for the Knights Templar, and some work for both. The are also innocent citizens who are ...
20
votes
4answers
3k views

A Knight or a Knave stood at a fork in the road

A competent logician was traveling home down an unfamiliar road. As he approached a fork heading in two different directions, he noticed a lone figure standing directly between the two routes. The ...
20
votes
1answer
1k views

Chess Connect Puzzle

I made up a new logic puzzle based on the games Numberlink and Chess, and thought it ended up rather interesting. The solution, as usual, is unique. The starting and ending positions of 7 chess ...
20
votes
1answer
805 views

Unbalanced weight of boxes

You are given a traditional balance scale with two pans (no reading) and 6 boxes with the following properties: 1) the weight of each box is a positive integer number 2) if you put any number of boxes ...
20
votes
2answers
2k views

Chess, for short people

Your grandfather and grandmother are playing chess, and you are sitting in the same room doing your own thing. A wide grin comes across your grandfather's face, and you can tell he is about to ...
20
votes
4answers
9k views

Determine the number $r$ from the statements

Alice and Bob are perfectly logical and super intellegent. Their professor decides to play a game with them, and he tells them: "I have chosen two numbers (integers) $x$ and $y$ with $2\le y \le x\...
20
votes
4answers
1k views

The Prison Escape Pt. 1 (THE DOOR)

You are currently escaping a prison owned by a mad scientist. As you were about to approach a large iron door that you think should be leading to the exit, a guard came out from the shadows. ...
19
votes
3answers
5k views

Too Many Damn Buffalos [closed]

"Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo" The sentence above is an actual sentence which makes perfect sense and is not wrong. The word "Buffalo" has three meanings: ...
19
votes
4answers
6k views

Who stole the flying car in Hogwarts?

During a trial in a Hogwarts court, Ron claimed that the Flying Ford Anglia car was stolen by Hermione. Then Hermione and Harry gave testimonies which, for some reason, were not recorded. Later on ...
19
votes
5answers
1k views

Single Colour Mastermind Problem

Imagine we're playing a game of mastermind with the usual eight colours, only this version has six holes across in which to place colours. And to make it easier for you I tell you that I have filled ...
19
votes
2answers
58k views

A man with two red pills and two white pills

Story: A Color blind man is travelling in a somewhat famous ship that had just a few people on board. Everything was smooth sailing for some days. One day,the man heard some noises from outside when ...
19
votes
6answers
3k views

Trapped in my Cellar

I have taken Doorknob hostage in my cellar. He is "perfectly" trapped - solid walls, solid floor, solid roof, no windows, etc. The only way out is a steel door and the only way to unlock the door is ...
19
votes
3answers
3k views

The origin of the blue eyes puzzle

The Blue Eyes puzzle is possibly the most popular induction-based puzzle known. Is it really known who was the first to come up with it (in any form). The link at xkcd says: I didn't come up with ...
19
votes
2answers
1k views

Heyawake: An Introductory Puzzle

This is a Heyawake ("divided rooms") puzzle. Rules of Heyawake: Shade some cells of the grid. Shaded cells cannot be orthogonally adjacent; unshaded cells must be orthogonally connected. ...
19
votes
3answers
2k views

Which two houses didn't get any mail that week?

Postman delivers the mail in the small village. This village has exactly ten houses, numbered from 1 to 10. In a certain week, he did not deliver any mail at two houses in the village but at ...
19
votes
4answers
2k views

Escape The Strange, Code-Filled Cave

You awake in a weird cave with something strange written on the wall. You find a small piece of paper on a rock. It says the following: Solving each code and entering it into the password entering ...
19
votes
3answers
2k views

Star Puzzle: Determine which circles are True and which circles are False

A circle with number x means: “This circle is connected (straight line) to x circles with a true statement” This is valid only if the statement in the circle is True. If a circle is False, ...
19
votes
2answers
572 views

Deer-hunting in Mesak

The following are ten sentences in the Mesak language: ...
19
votes
5answers
2k views

Find the 8-digit number

A secret number has each one of its 8 digits colored either blue or red. Here are some clues: 1. All digits from 1 to 8 appear in the number. 2. The first two digits are not consecutive. 3. The ...
19
votes
1answer
532 views

The Drouble with Doorples

You are the first officer aboard the Starship Enterpuzz, and moments after your captain departs for his week-long vacation, you discover that your ship is infested with tiny creatures called doorples. ...
19
votes
1answer
706 views

Numbered Logicians near the Blue Eyes island

There is an island near the famous Blue Eyes island where 100 perfect logicians live, each with a number between 1 and 100 (repetitions allowed) tattooed on their forehead. They know all the numbers ...
19
votes
1answer
5k views

Can you solve this logic puzzle in disguise? The color code behind the tomb

Another themed puzzle for you to enjoy. As with all my puzzles, please provided feedback about pro's and cons of the puzzle. The puzzle is self-contained (the link is just to provide a theme) and I ...
18
votes
9answers
1k views

One-one communication

5 people are standing in a circle - person A needs to tell his salary to person B, but none of the others should know A's salary. How is this possible, if every person only whispers to the person on ...
18
votes
6answers
3k views

Guess five binary digits!

Person A thinks of a 5 digit binary number. Person B tries to guess the number. B can guess a 5 digit binary number and A will respond with the number of correct digits (digits in the right place). ...
18
votes
8answers
3k views

What Imprisonment sentence will immortal citizen soldier Guilt receive?

On planet Immortal, everyone lives forever but essential resources (including food and supplies) on the planet are limited. Ultimatopian raiding swords are the most effective weapon available to loot ...

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