This tag is meant for questions about the mathematical principles behind games, riddles, or their possible solutions. If the answer is known to you please do not use this tag to "riddle" other users, but rather to ask about the correctness of a possible solution or ways to extend and improve an ...

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0
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4answers
135 views

Is there a theorem that disproves this or is this just some made up meaningless thing?

I find this slightly funny. I saw this on a meme:$$\begin{align}a=x\\ a+a=a+x\\ 2a=a+x\\ 2a-2x=a+x-2x\\ 2(a-x)=a+x-2x\\ 2(a-x)=a-x\\ 2=1\end{align}$$ How can these strange algebraic manipulations not ...
2
votes
4answers
385 views

Monty Hall problem vs. roulette systems - how are they different?

So I got interested in the Monty Hall problem - I understand what it's about, but somehow I can't wrap my head around the idea of the final choice not being 50/50. More precisely: we all know (or ...
2
votes
3answers
652 views

Use three 11's and various math symbols to make an equation equal to 6

The puzzle is to use the following symbols $$+,\;-,\;*,\;/,\;(\;,\;),\;!, \;\sqrt(\cdot)$$ in order to make a valid equation out of $$11~~~~~~11~~~~~~~11 = 6.$$ (There are three elevens with space in ...
2
votes
2answers
100 views

Tower of dice - Abstracting a practical problem to a mathematical method

This question arose when playing yahtzee with some friends. Not entirely sure if I'm in the right area, but hope you can help. How many dice do you need to create a tower whose walls have the same ...
2
votes
4answers
583 views

balance scale problem for 13 (not 12) items

The 12-item balance scale puzzle is very familiar. The object is to find the lone non-standard item (if one exists) out of a group of 12 seemingly identical items, using a balance scale and a maximum ...
18
votes
7answers
2k views

“How long 'til we get there?” Road trip puzzle

Road trips can be fun, but they often appear to go slower the closer you get to your destination. I thought up this puzzle while on a recent trip. Thought it would be good food for thought. Curious ...
5
votes
5answers
441 views

Puzzle: Dropping balls along the way

A man has some balls in his pocket. Let the number of balls in his pocket be $n$.(Consider $n$ as an integer. If any decimal value occurs, consider its floor value. For example, if $n$ = 2.6 then take ...
6
votes
0answers
141 views

Card passing game, maximum length

Quoting from this question: There are 25 people sitting around a table and each person has two cards. One of the numbers 1,2,..., 25 is written on each card, and each number occurs on exactly two ...
3
votes
4answers
405 views

Numerical puzzle

I'm stuck here with some numerical rebus - Given: $A^2=BC, A^3=CA$ Find: $A+B+C$ $13$ $12$ $11$ $10$ (only one correct solution) Note that letters represent digits. I can't think of any idea ...
2
votes
2answers
334 views

Numerical rebus

Given: ${AA}\times{BC}=BDDB$ Find $BDDB$: $1221$ $3663$ $4884$ $2112$ The way I solved it: First step - expansion & dividing by constant ($11$): $AA\times{BC}$=$11A\times{BC}$ $1221$ => ...
4
votes
2answers
286 views

Cut the rope puzzle

This question was asked to me in an interview, I still cannot think of its solution. Can anyone help? Following is the question: Given an infinite number of ropes of length $R$, you have to ...
0
votes
3answers
630 views

knight and knave problem

For this question, suppose you are on the island of knights and knaves. Remember that knights always speak truth while knaves always tell a lie. (a) Suppose you come across two of the natives. You ask ...
16
votes
1answer
964 views

Given a set of digits, what is the biggest number we can make using exponentiation - numberphile noodle quiz

The question is motivated by a question on a can of number noodles. Each item is a digit between $0$ and $9$. Clearly, if you form a string and consider it to represent a base $10$ integer, then ...
2
votes
2answers
307 views

Prove or disprove a chessboard with diagonal corners removed, cannot be tiled with L shape pieces or size 2

I think this is impossible, but I don't know how to prove an integer solution doesn't exist for a given equation. Here's my approach: First, observations: The removed tile will be of the same color. ...
3
votes
2answers
323 views

A less challenging trivia problem

There are 25 people sitting around a table and each person has two cards. One of the numbers 1,2,..., 25 is written on each card, and each number occurs on exactly two cards. At a signal, each person ...
-3
votes
2answers
155 views

Find the last number in the given sequence

Find the last number in the given sequence $$\begin{pmatrix} 4& 9& 20\\ 8& 5& 14\\ 10& 3& ?\end{pmatrix}$$ (It's $3\times3$ matrix)
2
votes
1answer
524 views

Puzzle on the triangle.

In triangle top four figures that have to be repositioned to form the "triangle" without a unit square. How to explain this? Thank's.
8
votes
3answers
201 views

Group of sphere transformations, impressing friends

Ok, so here's the story: I am reading a book on algebra and, via some exercises, discovered that in any group $G$, the order of $x \cdot y$, written $o(x \cdot y)$, equals $o(y \cdot x)$. Now, this is ...
25
votes
7answers
7k views

Is there no solution to the blue-eyed islander puzzle?

Text below copied from here The Blue-Eyed Islander problem is one of my favorites. You can read about it here on Terry Tao's website, along with some discussion. I'll copy the problem here as ...
2
votes
2answers
319 views

Four men seated in a boat puzzle

I am looking for an elegant way to solve this rather simple logic puzzle using mathematical logic (statements, conjunctions, disjunctions, implications, tautologies, predicate logic and so on). I am ...
6
votes
1answer
893 views

Find volume of crossed cylinders without calculus.

I found this puzzle here. (It's labeled "crossed cylinders".) Here's the description: Two cylinders of equal radius are intersected at right angles as shown at left. Find the volume of the ...
6
votes
2answers
2k views

topology puzzle - without cut the rope, separate two rings

hello I wonder whether this puzzle is possible to solve. if possible, what kind of thing should I learn to solve this? the problem is make left one to right one without cut the rope only stretch and ...
0
votes
1answer
58 views

homework cheyenne

It is a four-digit number. Its largest place value is its one place. The squares of each of its digits are less than nine. Its thousandths didgit divided by its ones digits is 50%. The sum of two pf ...
0
votes
1answer
866 views

three girls gave a friend 60 rupees and asked her to buy them a kilo of mangoes from the market. [duplicate]

Possible Duplicate: Riddle (simple arithmetic problem/illusion) three girls have a friend 60 rupees and asked her to buy them a kilo of mangoes each from the market. the mangoes cost 55 ...
1
vote
1answer
277 views

Setting A Paper on Mathematical Puzzles

I need to set a paper for High School Students on Mathematical Puzzles which make the use of logic, simple combinatorics and algebra. Can people provide new and innovative questions. The questions ...
6
votes
1answer
392 views

“8 Dice arranged as a Cube” Face-Sum Problem

I found this here: Sum Problem Given eight dice. Build a $2\times 2\times2$ cube, so that the sum of the points on each side is the same. $\hskip2.7in$ Here is one of 20 736 ...
7
votes
2answers
3k views

Father and daughter river crossing puzzle.

There are 3 men and 3 girls. lets name them ABC, A's daughter X, B's Daughter Y, C's daughter Z. Rule to cross the river: At once Only two Can go on boat. Only Men know how to drive the boat (So ...
2
votes
1answer
123 views

Books in the Library of Babel

I recently came upon a comment in the mathematics overflow stating that the infinite string of pi was similar to The Library of Babel. This library is a universe containing every possible combination ...
2
votes
2answers
142 views

Puzzle on Ranks

Here is a puzzle from textbook : 40 Puzzles and Problems in Probability and Mathematical Statistics Peter draws n = 100 independent realizations of a continuous rv and ranks them in increasing order ...
4
votes
1answer
187 views

What function does this infinite series represent?

$$\frac14+\frac{x-4}{2!x^2}-\frac{(x-4)(2x-4)(3x-4)}{4!x^4}+\frac{(x-4)(2x-4)(3x-4)(4x-4)(5x-4)}{6!x^6}\mp\ldots$$ Can anyone deduce the sum of this series? The reason I ask is because I made it and ...
1
vote
1answer
177 views

Winning single-pile, variable limits Nim [duplicate]

Possible Duplicate: Winning strategy for a matchstick game The rules of this variant of Nim are as follows: Starting at zero, each player counts up between 1-N numbers. The person that ...
1
vote
1answer
3k views

How Old Are Children?

Two old friends, Jack and Bill, meet after a long time. Jack: Hey, how are you man? Bill: Not bad, got married and I have three kids now. Jack: That’s awesome. How old are they? Bill: The product of ...
8
votes
3answers
2k views

How Strong is an Egg?

You have two identical eggs. Standing in front of a 100 floor building, you wonder what is the maximum number of floors from which the egg can be dropped without breaking it. What is the minimum ...
0
votes
1answer
59 views

Deduction from a sequence of statements

check the validity of the following argument: "if the rents of hotels in jammu are fixed or the prices of the commodities are reduced then the income of bussinessmen shall decrease. If the income of ...
2
votes
4answers
901 views

A coin-weighing puzzle with 80 coins

In 80 coins one coin is counterfeit. What is minimum number of  weighings to find out counterfeit coin? PS: The counterfeit coin can be heavy or lighter.
2
votes
2answers
241 views

Number of songs sung.

There were 750 people when the first song was sung. After each song, 50 people are leaving the hall. How many songs are sung to make them zero? The answer is 16, I am unable to understand it. I am ...
15
votes
4answers
8k views

How is this a number sequence 58, 26, 16, 14, 10

I recently had a IQ Test taken and we all got stuck on the same question. The question was: What comes next in the following sequence? 58, 26, 16, 14, _ The answer given in the answer sheet was 10. ...
1
vote
1answer
649 views

How to solve multiplication alphametics?

I am referring to puzzles like these, where every letter represents a unique number (0-9): ...
1
vote
2answers
3k views

How to solve this alphametic (verbal arithmetic)?

I know I can get the answer for this puzzle but I'm struggling to see how to solve it. Every letter represents a different number (0-9): ...
2
votes
2answers
570 views

How do I find the center of a circle on a rectangular piece of paper?

I have a rectangular piece of paper with a circle printed on it. I also have a handy-dandy writing utensil. How can I locate and mark the center of the circle? Here's some technicalities: The ...
3
votes
2answers
253 views

Rotating the graph of a function

I think I have answered the following puzzle: Rotations Under what conditions can you rotate the graph of a function about the origin, and still have the resulting graph being the graph of a ...
1
vote
2answers
112 views

Differing by $2$ “puzzle”

I've been thinking about the following: For (1) I drew $y= 0$, $y=2x$ and $y = 2x$, $y = 4x$. The expression I wrote down if $y = mx + b$ is one of the lines then $y_2 = (m \pm 2)x + b$ is the ...
7
votes
1answer
162 views

“Multi-facets” rope puzzle

I've done the following, can you tell me if it's correct? If $n$ is the number of sides of the rope and $k$ is the number of rotation, e.g. $k=0$ for glue each side to itself then I think the ...
1
vote
1answer
244 views

Latin Squares - Proving the Unique number of Sudoku that can be generated

I recently read that the Sudokus are just Latin Squares for $n = 9$. I know that proving the number of Latin Squares is considered difficult to generalize in terms of $n$. I would like to know if ...
1
vote
2answers
332 views

Mystery card probability riddle [duplicate]

Possible Duplicate: The Monty Hall problem I’m looking for the better explanation to the solution of this riddle, because I always struggle to get people to accept the ones I know. Riddle: ...
4
votes
1answer
110 views

How old are they?

One man speaking to other: When you were born I was 20 years old. Today I am twice the age you were when I was the age you are now. What are the ages of both of them?
84
votes
17answers
14k views

How do you find the center of a circle with a pencil and a book?

Given a circle on a paper, and a pencil and a book. Can you find the center of the circle with the pencil and the book?
2
votes
7answers
4k views

Simple Logical reasoning question

I am trying to improve my logical reasoning skills. Came across below question. See image. Can anybody let me know what the logical reasoning to the answer will be. I have no idea how to solve this ...
4
votes
4answers
580 views

The concept of infinity

This evening I had a discussion with a friend of my about a mathematical riddle and the concept of 'infinite' The riddle Imagine a hotel with an infinite amount of rooms, and all of the rooms are ...
2
votes
1answer
277 views

Nullarbor Plain puzzle

I have the following puzzle I'm trying to solve: "A man lost on the Nullarbor Plain in Australia hears a train whistle due west of him. He cannot see the train but he knows that it runs on a very ...