All Questions

Filter by
Sorted by
Tagged with
6
votes
3answers
512 views

Magic back yards

My back yard forms a rectangular grid of squares except some of the squares are missing as they are covered by pipes or a small tree. The layout is as follows. A 'x' indicates a square that is free ...
10
votes
6answers
2k views

Miss Danner and the Elmer Boys

Miss Danner, a primary school teacher, is on her way to school - it's the first day of school this year. In class, she sees two identically looking boys sitting in the first row. Aw, twins, so cute!...
22
votes
12answers
158k views

Four prisoners wearing black and white hats

There are four prisoners. All four prisoners will be freed, if at least one of them correctly guesses the color of the hat on his head. They can't speak to each other, and they can't touch each other....
9
votes
1answer
347 views

What factors affect the difficulty of Hamiltonian puzzles?

There is a type of puzzle which can be modelled as finding a Hamiltonian path through a graph. One example is the Icosian game. What factors affect the difficulty of such puzzles, to be solved by ...
23
votes
2answers
11k 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 ...
47
votes
11answers
284k views

6, the magic number

Here's a fun (albeit difficult) one: Make these equations true using arithmetic operations: ...
8
votes
2answers
609 views

Pattern 1 — Numbers

Fill in the missing number: 10, 30, 60, 01,__, 12 Use a bit of lateral thinking. And math. Also, I do know the answer. Just giving the question as a teaser.
0
votes
3answers
47k views

Candle timing measurement [duplicate]

You are given two candles of equal size, which can burn 1 hour each. You have to measure 90 minutes with these candles. (There is no scale or clock). You are also given a lighter.
9
votes
5answers
484k views

Puzzle of putting numbers 1-9 in 3x3 Grid to add up to 15

In a 3x3 grid, I'd have to put numbers from 1 to 9 in a manner so that respective row, column and diagonal add up to 15. I have only been able to come up with one solution: 6 1 8 7 5 3 2 9 4 ...
8
votes
2answers
654 views

Epimenides paradox

Epimenides (a Cretan) once wrote a poem, in which he stated that all Cretans are liars. Since he is a Cretan, and therefore a liar, Cretans are veracious. But then again he wouldn't be a liar! This ...
13
votes
2answers
1k views

How to survive, with statistics

You are kidnapped and your offender plays a game with you. In front of you are 2 boxes, containing a total of 50 white balls and 50 black balls. The kidnapper will pull out a ball from one of the two ...
7
votes
1answer
523 views

Number of true sentences

You are given a sheet of paper with $100$ sentences, in the form of: There are exactly $x$ wrong sentences on this sheet. There is exactly $1$ wrong sentence on this sheet. There are exactly $2$ ...
1
vote
1answer
662 views

3 Switches identification [duplicate]

Picture this. You are in a hallway of a very large office building. Near you are 50 light switches of which 3 are unlabled. These lightswitches are connected to 3 lights in 3 separate rooms. The ...
2
votes
10answers
5k views

The cage - a logic puzzle

One summer day John was jogging in the countryside wearing only his normal running clothes (shorts, vest, sports shoes and socks). After running for some time he saw that the lace of one of sports ...
4
votes
2answers
636 views

What's the largest number that can be obtained in the process of solving a numbers game on Countdown?

Countdown is a British game show in which people try and make the longest possible word out of nine given letters, reach a target number using six given numbers and basic arithmetic, and then solve a ...
8
votes
2answers
4k views

Spatial IQ test puzzles

The following puzzles were created by Robert Lato to measure IQ in range between 160 and 190. Whats the solution to them? You are supposed to draw in the dented boxes what the missing figure is for ...
5
votes
1answer
2k views

Is there a general strategy for decoding Brainfuck mutations?

There are many Brainfuck variations used in puzzles in geocaching, for example Ook! They are usually not so hard to decode because they are 'standards' - therefore the substitution is googlable, there ...
4
votes
3answers
348 views

How many subset intersections do you require?

Alice chooses a subset S of {A,B,C,D,E}. Bob makes a guess of any subset T and Alice tells him the number of elements in S$\cap$T. Bob has to continue making guesses until he can exactly determine S....
5
votes
1answer
367 views

Is there a second solution to points and perpendicular bisectors problem?

There is a problem from M.Gardner book ("Wheels, Life, and other Mathematical Amusements", p. 201): [Hallard T. Croft] asked if there existed a finite set of points on the plane such that the ...
5
votes
2answers
431 views

Land claim staking

It was traditional during the times of the Oregon Trail and the gold rush that men founding new towns were allowed to stake their claim on land upon arriving to the west coast. Suppose a man is ...
9
votes
2answers
717 views

Middle weight puzzle

There are 100 white and 101 black balls. All the balls have different weights. Balls of the same colour look the same. White balls are numbered from lighter to heavier (from 1 to 100). The same is ...
-3
votes
1answer
1k views

Can anyone explain me how to solve this problem? [closed]

Two workers A and B manufactured a batch of identical parts. A worked for 2 hours and B worked for 5 hours and they did half the job. Then they worked together for another 3 hours and they had to do (...
6
votes
2answers
7k views

Remove first or last letter - What is the name of this word puzzle/type of word?

When I was a child I had a puzzle book. One of the puzzles involved finding words where you can remove one letter from the beginning or end of the word, and you still have a valid word. You can ...
11
votes
6answers
7k views

Who has mud on their forehead?

This is probably my favorite puzzle. It uses logic, but it's not one of those "logic table" puzzles which i dislike immensely. What is this type of puzzle called and are there more like it ...
8
votes
1answer
595 views

Find the smallest N to complete the task: “Write a true self-reflective statement about the digits from 0 to N”

Following this question, consider the case where we change the number of digits and formulate the task like this: Write the statement like "This statement contains $M_0$ 0's, $M_1$ 1, ..., $M_N$ N'...
7
votes
2answers
770 views

Write a true self-reflective statement about the digits from 0 to 9 (or prove it can't be done)

This statement contains 3 0's, 1 1, 4 2's, 1 3, 5 4's 92 5's, 6 6's, 53 7's, 58 8's, and 9 9's. Clearly that statement is incorrect. It only has 1 occurrence of ...
12
votes
2answers
3k views

A two-player game

This is from the current weekly math challenge from the newspaper Le Monde. Alice and Bob play the following game, in turn: A number between $1$ and $10$ is written on a blackboard. At each of their ...
7
votes
3answers
1k views

Most moveable chess position

What would be a chess position, where a player (white or black) has a maximum possible number of different moves to make? There are two options: 1. The position must be possible to obtain in a ...
11
votes
3answers
2k views

How can I spot an X-Wing?

I was doing a Sudoku puzzle (in my morning newspaper on the train to work) and got stuck at this point: ...
3
votes
3answers
13k views

Task 39 from iqtest.dk

What is the solution for this puzzle from this IQ test?
7
votes
4answers
7k views

What is the pattern in this IQ test puzzle with spotted triangles and arrowed circles?

The solution for this test is 1. But what is the pattern? P.S. I checked that 1 is the answer by simply trying different options in the test itself (Russian site for the test is here: http://sly2m....
6
votes
1answer
795 views

What's the more general case of this puzzler?

There is a puzzle which I remember from many years ago which can be summarised as It is easy to arrange 20 items in five rows of four items, but how could you arrange just 10 items in five rows of ...
32
votes
18answers
50k views

Draw a line through all doors

I saw the following problem on 4chan and couldn't solve it: It's very likely to be some kind of troll (no solution). I'm hoping to see some rigorous proofs that disprove the existence of such a line....
13
votes
5answers
3k views

What is the smallest positive integer, which can not be written without repetitions of digits and using arithmetics only?

Suppose you are allowed to use all 10 digits (0,1,2,...9) at most once each; 4 arithmetic operations ($-$,$+$,$\times$,$\div$), each any number of times; parenthesises to group operations; and you can ...
9
votes
3answers
2k views

Stranded Nomad Riddle

Background Story A nomad is injured in the desert. And he can't move, but fortunately an oasis is nearby, and he can stay there for enough time until rescue arrives. But the water is a magical water, ...
8
votes
2answers
499 views

Drawing lines on a map

This was a puzzle that appeared in an old math contest as well as an unrelated puzzle book. Suppose we have a two-dimensional map with a lot of cities marked on it. For each city on the map, we ...
6
votes
4answers
3k views

To be a bookworm

I have a complete set of encyclopaedias on my bookshelf, beautifully bound and neatly arranged with volume 1 on the left and volume 32 on the right. They were a talking point when I bought them 30-odd ...
25
votes
5answers
3k views

How much red sand and how much blue sand?

This is a puzzle I remember arguing with my sister about when we were little... (Not quite as loud (or as public!) as the Monty Hall argument we had.) You have two large beakers of the same size. ...
2
votes
2answers
930 views

SuDoKu Unique Rectangles and the “2 blocks” rule

On HuDoKu's page on Uniqueness, under Unique Rectangle, it says A common mistake when looking for URs is to violate the "2 blocks" rule (see the Sudopedia article for an example). Unfortunately, ...
14
votes
5answers
2k views

Church charity lottery

Suppose you've rigged the church charity lottery so that you cannot lose (what a swell guy you are). There are 10 drawings, and you start off with 10 dollars exactly. At each drawing, you must chose ...
3
votes
1answer
925 views

Paying the unfair Troll toll

Here is more complicated Paying the Troll toll like puzzle: Between you and your destination, you have 7 bridges, and there is a troll under every bridge. Each troll, quite rightly, insists that ...
2
votes
1answer
850 views

How long is needed to clear the area of robots?

This question has been written in order to help answer Fastest way to collect an arbitrary army. There is a 1 x 1 square area with corners labelled clockwise A, B, C and D. A finite number of ...
3
votes
1answer
456 views

$N_{max}$ balls in 3 weightings

This is a small generalization of well known 12 balls puzzle: You are given two-sided scale and some number of balls. All balls have the same weight but one. It is of a different weight, although ...
20
votes
3answers
2k views

An arithmetic puzzle from 1645

The following problem is found in the first Norwegian arithmetic, published in 1645 by Tyge Hansøn: Three hundred oxen large and small, A cattle owner wanted to buy: 3 for 63 daler he got. ...
5
votes
2answers
2k views

I have a pattern problem to solve!

The question reads: From the given pattern, What comes in place of '?'
14
votes
7answers
3k views

Twelve balls on a scale, where one ball is lighter and another is heavier

Suppose you have twelve identical-looking balls. Ten of them are the same weight, but one is slightly lighter and one slightly heavier, in such a way that the weight of the lighter and heavier ball ...
56
votes
3answers
254k views

What is the number of the parking spot?

One needs to find the number under the car.
5
votes
1answer
657 views

Maximum time for ants to fall off stick (with a non-instant turning speed)

This question is purely a follow-up to Maximum time for ants to fall off stick. Suppose there are $n$ ants on a stick which has length 10. At any time, the ants may be facing left or right, the ...
32
votes
3answers
5k views

Maximum time for ants to fall off stick

Suppose there are $n$ ants on a stick which has length 1. At any time, the ants may be facing left or right, the initial directions of the ants are arbitrary. Each ant can be modeled as a point in the ...
8
votes
10answers
7k views

Find the missing number: A Pentagonal pattern puzzle

This question is based upon the figure shown below. $$ \begin{array}{|ccc|c|ccc|c|ccc|} \; & 41 & \; & \; & \; & 31 & \; & \; & \; & ? & \; \\ 5 & \; ...

15 30 50 per page
1
446 447
448
449 450
452