# Tagged Questions

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### The number of ways people standing in a line can be holding hands

I'm writing a program to analyze the maximum unique sequences of data in a string, given certain sets of two can be interpreted in two ways. There's a bit of math that I can't figure out, I've ...
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### Numbering edges of a cube from 1 to 12 such that sum of edges on any face is equal

Assign one number from 1 to 12 to each edge of a cube (without repetition) such that the sum of the numbers assigned to the edges of any face of the cube is the same. I tried a bunch of equations but ...
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### Number of attempts needed to open lock

There are $3$ knobs for a lock $A,B,C$. Each can take $8$ positions, and for each knob there is one correct position. When $2$ of the knobs are at their correct positions, the knob opens (irrespective ...
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I have n sets having values less than 100. I need to find how many arrangements could be made if I pick one element from each set such that in the given arrangement there are no duplicates? NOTE: A ...
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### Chessboard problem in IMO2014

This is the second problem on the IMO2014 problem list: Let n $\ge 2$ be an integer. Consider an $n \times n$ chessboard consisting of $n^2$ unit squares. A configuration of $n$ rooks on this ...
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### Extended Calendar Cube Question

The calendar cube puzzle is famous: using two six-sided cubes, label them such that any day of any month can be represented by positioning the cubes accordingly. The solution involves allowing the ...
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### Lexicographical rank of a string with duplicate characters

Given a string,you can find the lexicographic rank of a string using this algorithm: Let the given string be “STRING”. In the input string, ‘S’ is the first character. There are total 6 characters ...
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### how many words can be formed using all letters in the word EXAMINATION

Assuming any sequence of letters is a word, how many words can we form in such a way that the first two letters are different consonants while the last two letters are vowels?
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### A riddle with a witch and some gnomes

My question concerns a variation and a generalization of the following riddle. The Original Riddle: A wicked witch kidnaps 2 gnomes. She paralyzes them, and places a hat on each of their heads. Each ...
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### Blocking lines of length $5$ in a $7 \times 8$ matrix; how can we know the solutions have a specific form?

A friend shared with me the following puzzle he encountered in a Chinese math competition: In a $7 \times 8$ matrix, we place tokens so that any straight line of length $5$ (horizontal, vertical, ...
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Suppose we have a house in which every room has an even number of doors. Prove that the number of doors from the house to the outside world is also even.
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### Drawing previously undrawn cards from a deck

Suppose you have a deck of $y$ cards. First, randomly select $y-x$ distinct cards and sign the face of each, then shuffle all the cards back in to the deck. Proceed as follows: Draw a card. If it is ...
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### Two children paradox : where is my reasoning wrong?

I hope here is the good place to be asking this. Apologies otherwise. The statement is as follow : "Ms Michu has two children. We know one of the two is a girl, we call that girl Ludivine. What is ...
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### History of a combinatoric problem: exchanging numbers by throwing stones

Another user recently asked a question on the Puzzling stack: Two spies throwing stones into a river. Suitably generalised, it becomes: Two spies (Alice and Bob) need to exchange a message. Each ...
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### Number of tries to guess M-1 letters from M-letters-code.

There are N letters in an alphabet. There is a combination lock, the code to it consists from M different letters. You can input M letters combination to try to open the lock. If you guess at least ...
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### When to be sure that we have counted all the squares in such problems [duplicate]

My first question is: How would one solve such problems (in general,squares+rectangles). What should be the general technique?How can this problem be reduced to a mathematical problem? My second ...
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### Chords of a 20-gon

Twenty points lie on a circle, so as to form a regular polygon. Then they are split into ten pairs, and the points in each pair are connected by a chord. Prove that some pair of these chords have the ...
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### Number of solutions to sudoku puzzle

Inspired by this question, consider hints on a Sudoku board. A regular puzzle has a unique solution. It is clear that there are puzzles with 2 or 3 solutions, and therefore, I guess, puzzles with say ...
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### Computing probabilities of consecutive letters in a word grid

I'm sure most people are familiar with word grid games like Boggle and the newer digital versions Scramble with Friends and Ruzzle. For anyone not familiar, the idea is to find words by using ...
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### Maximal size of set cover

Let $S$ be a set of size $1983$, and let $A_1,..,A_k$ be a familiy of subsets of $S$ such that: The union of every 3 sets of the family is S. For every pair of sets the union of them contains no ...
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### Sudoku puzzle with exactly 3 solutions

While published sudoku puzzles typically have a unique solution, one can easily conceive of a sudoku puzzle with two solutions. However, is it possible to construct a sudoku puzzle with exactly 3 ...
223 views

### Deducing correct answers from multiple choice exams

I am looking for an algorithmic way to solve the following problem. Problem Say we are given a multiple choice test with 100 questions, 4 answers per question (exactly one of those four being ...
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### Rooks in 3D chess board

How many rooks are needed for a 3D chess board of size NxNxN so that every empty cube on the board can be reached by a rook in a single move?
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### 52-card trick for a larger deck?

Long ago someone demonstrated the following card trick with a standard 52-card deck: (1) A volunteer selects 5 cards from a shuffled deck, which the performer does not see. (2) The assistant puts ...
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### Number-Theoretic Coin Puzzle

There are three piles of coins. You are allowed to move coins from one pile to another, but only if the number of coins in the destination pile is doubled. For example, if the first pile has 6 coins ...
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### How many triangle can be drawn with those points? [duplicate]

There are 7 points on the circumference of a circle.How many acute triangle can be drawn with those points. please help me to solve this problem.
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### Maximal number of kings on a chessboard, but this time two can be adjacent.

How many kings can be placed on an $8 \times 8$ chessboard such that every king can capture (is adjacent to) at most one other king? I can do 26, but can not prove that this is optimal.
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### Maximum score for the game

Here is a game: There is a list of distinct numbers. At any round, a player arbitrarily chooses two numbers $a, b$ from the list and generates a new number $c$ by subtracting the smaller number from ...
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### How do I find the maximum number of knights on a chess board?

I came across this problem and after thinking a lot I could not get any idea how to calculate it. Please suggest to me the right way to calculate it. Given a position where a knight is placed on ...
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### Optimized search for lock combinations

I came across an interesting puzzle the other day expressed as follows. You have a combination which has a dial on its face with the values of {1-30}. The combination that will open the lock is an ...
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### Kings on a chessboard

In how many different ways can six kings be placed on a $6\times 6$ chessboard so that no one attacks the others? If the problem was asked for a $3 \times 3$ board and $3$ kings, then the answer ...
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### TicTacToe with considerations of symmetry

It is not difficult to determine the number of possible games of tic toe, but what about when rotationally symmetric positions are considered equal? Please do not simply give me the number, I would ...
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### probability of a word in a string

What is the probability of a word n characters long appearing in a string of m characters, in an alphabet of x characters? A word here is simply a string of characters contained in another string of ...
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### Coloring 5 Largest Numbers in Each Row and Column Yields at Least 25 Double-Colored Numbers

This is a question from a very old American Mathematical Monthly, if I recall correctly. It has a very nice solution and illustrates an often useful technique. If it is unsolved after a while, I will ...
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### A “What's my vector?” game

Alice chooses a binary vector $V$ of length $n$ which is unknown to Bob. In each round Bob can choose any number of indices $i$ and then Alice tells Bob how many of the $V_i$ are set to $1$. The ...
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### How many unique patterns exist for a 5x5 grid with paths of spaces intersecting at 1 space and leading to each edge of the grid?

I'm try to design a game in which the board is made up of a 3x3 grid of square tiles. Each tile is a 5x5 grid of spaces. Each tile has 4 exit spaces each located on 1 of the middle 3 spaces along ...
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### Combinatorics: Lock puzzle , minimum combinations

A safe has three locks of which every lock has 8 possibilities 1, 2 ...8. Safe gets opened if any 2 of 3 locks gets opened. So, a possible way to open safe is try 2 locks, for each possible pair of ...
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### What is the minimum number of locks on the cabinet that would satisfy these conditions?

Eleven scientists want to have a cabinet built where they will keep some top secret work. They want multiple locks installed, with keys distributed in such a way that if any six scientists are present ...
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### Cutting a hexagon to make an equilateral triangle

The problem is to cut a regular hexagon into parts that can be put together (without overlaps or wasting any parts) to make an equilateral triangle. The cuts should all be straight. What is the ...
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### Minimum Overlap

You have a set of ten numbers, and you are trying to cover all 4-element subsets of this set. To do this, you choose 5 elements from the set every time and you cover all 4-element subsets of your ...
Let $p_n$ be a pairwise partition of $\{1,2,...,2n\}, n\in \bf N$ where $(a,b)\in p \implies a<b$, and $P_n$ the set of all such pairwise partition. \$d(n) := \min_{p_n\in ...