Questions tagged [combinatorics]
A puzzle based on combinatorics, which is the study of counting discrete structures. Use with [mathematics]
720
questions
8
votes
1answer
873 views
How many "mathletic" couples were having dinner at the table?
Once, a number of couples, each one of them happening to be composed of a mathematician and an athlete (hence 'mathletic'), wanted, in order to diversify communication, to sit down at opposite sides ...
5
votes
5answers
492 views
Find the most unfortunate compact combination of coins to have in LOLandia
You live in LOLandia. Its currency is called 'lulz' and comes in the form of coins and paper banknotes. The smallest paper banknote has a nominal value of 500 lulz. There are six types of coins, each ...
4
votes
1answer
211 views
My High School's Reunion
My high school is celebrating 30 years since graduating its first class and is planning to invite for lunch 20 alumni, 600 in all, from each of those classes.
Hosts are planning to sit everyone in ...
7
votes
2answers
290 views
Integers containing all ten digits
It is known that most positive integers contain at least one copy of each of the ten digits.
What is the largest n such that at most 50% of the integers in the set [1,2,3,...,n]
contain at least one ...
7
votes
1answer
309 views
Can you distribute the balls equally into 2 boxes?
You have 2 boxes and an even number ($2n$) of balls in the first box. Your goal is to distribute the balls equally into the two boxes, so that each box contains $n$ balls. You must obey the following ...
-2
votes
2answers
148 views
How to solve this problem on overlapping?
In cases of problems involving order and ranking where there are two indices (namely left and right) there is a particular chance of overlapping. Let us take an example to justify this:
Ranjan is ...
32
votes
7answers
2k views
Two out of a dozen cartons have Easter eggs. Two people try to find one Easter egg carton, each using a different strategy. Who is expected to win?
I have found a counter intuitive puzzle. I have read the answer given at the source and understand it completely. But, what I am unable to understand is why my intuition turned out to be wrong. ...
10
votes
2answers
621 views
Two dimensional Mastermind
You have probably played the classic game of Mastermind with 4 pegs and 6 colours. It turns out that the codebreaker can always find the pattern in 5 moves or fewer.
Now consider the 2D version of the ...
16
votes
2answers
699 views
XV Sawtooth Sudoku
Please find below a variant Sudoku puzzle, based on a combinatorics problem I was having a look at. The timing is right, as I recently saw @BeastlyGerbil back in chat, and I know that user is a big ...
8
votes
2answers
511 views
Largest rectangle from 20 Lego bricks
You have twenty 2x4 Lego bricks, like the one shown below
What is the area of the largest rectangle you can make satisfying the following conditions:
All bricks must be connected in a single ...
7
votes
2answers
609 views
5x5 binary grid with every 2x2 sub-grid occurring once
Can you paint a $5 \times 5$ grid in two colors, such that each of the $2 \times 2$ possible sub-grids ($2^4 = 16$ combinations) occurs exactly once in the grid?
3
votes
2answers
220 views
2x4 grid with distinct differences
Can you place numbers from the range $[0,16]$ into a $2 \times 4$ grid such that all orthogonal pairwise differences are distinct? In other words, we want every pair of numbers that lie in the same ...
9
votes
4answers
3k views
6x6 Minesweeper grid with all threes
Can you place 16 mines on a 6x6 Minesweeper grid such that each number produced is a 3? Bonus: can you find multiple solutions that are not rotations or reflections of each other? Good luck!
Related ...
-6
votes
1answer
78 views
Overlapping in Order and ranking
For order and ranking questions there are a couple of the questions which require to find the total number of persons along with maximum and minimum condition which is difficult for me to comprehend. ...
6
votes
1answer
636 views
Paint Eleven Squares
I was inspired by this great question: Paint Eight Squares
Given a $5 \times 5$ grid of white squares, can you paint 11 of the squares black so that each white square is orthogonally adjacent to ...
4
votes
3answers
303 views
Special team in a soccer tournament
$N$ teams play in a soccer tournament where each team plays every other team exactly once. A game has 3 possible outcomes: team 1 wins, team 2 wins or a draw.
Is it possible that one team achieves ...
9
votes
9answers
3k views
How many gold coins can you extract from the billionaire?
An eccentric billionaire plays a game with you. She has an urn with 100 gold coins.
Each time, you can take any number of coins from the urn.
If you take n coins, she will flip a fair coin.
If head, ...
5
votes
3answers
432 views
Generalization of the two-surgeons-two-patients-and-two-gloves puzzle
This is the original puzzle with $n=2$. I recommend solving it before this one to get acquainted with the mechanisms.
There are $n$ patients in an hospital (let's call them $p_1 \dots p_n$), each of ...
9
votes
2answers
178 views
A grid-line of nuclear balls
Imagine a semi-infinite grid-line in which every box can hold any number of balls.
...
6
votes
1answer
379 views
Discounts in a shop
I came across this sign in a shop and thought it could make a nice puzzle. So you can buy items and get discounts depending on how many items you got. You can combine discounts and use as many as you ...
2
votes
1answer
199 views
Balls in baskets
There are 16 baskets: 4 red, 4 blue, 4 green and 4 black. Each basket contains a ball from one of the 4 colours (see image). You can pick up a ball from one basket and swap it with a ball from another ...
4
votes
1answer
123 views
combinations of a strange magic square
The following question has been described to me by my math teacher:
The diagram below is to be filled in so that each white square contains a different whole number from 1 to 12 (inclusive) and the ...
5
votes
3answers
285 views
The vaccine distribution conundrum
The context is that of a pandemic that is spreading wildly and requiring global vaccination of the population.
You are working in a distribution center for the vaccines.
One day you have been ...
20
votes
2answers
1k views
Beans under the chessboard
Under every grid cell of a chessboard, I put either one bean or nothing.
Now if you choose a (grid) rectangular area on the chessboard, then I will tell you the parity of the number of beans under ...
21
votes
3answers
3k views
Can you survive this infinite zombie attack?
You're surrounded by infinitely many zombies. You're at the origin, and zombies occupy the points $(100i,100j)$ for all integer $i, j$ except the origin, as shown below:
You and the zombies move ...
3
votes
2answers
683 views
Too many school assignments
This year we have to make our school assignments in pairs.
With each classmate must be made exactly one of those assignments.
Exactly 30% of the assignments will be made by a pair of girls.
How many ...
14
votes
2answers
835 views
7x7 Golomb square
Can you paint $7$ cells of a $7 \times 7$ grid such that the Euclidean distance* between any pair of painted cells is distinct? Good luck!
*The Euclidean distance between cells $(r_1,c_1)$ and $(r_2,...
9
votes
1answer
749 views
Who will win in a game of writing 3 consecutive Xs on a 2022 × 1 board?
Ana and Bob alternately write Xs on a 2022 × 1 board. The winner is the one who makes 3 consecutive Xs. Who has the winning strategy if Ana plays the first move? Describe such a strategy.
3
votes
1answer
132 views
Snake game on a 9×9 grid
You are playing a snake game. The snake starts in the top-left corner of a grid. Each cell of the grid is either empty or a wall. Each turn you can press a key to move the snake in one of four ...
2
votes
1answer
112 views
Snake game on a 6×6 grid
You are playing a snake game. The snake starts in the top-left corner of a grid. Each cell of the grid is either empty or a wall. Each turn you can press a key to move the snake in one of four ...
4
votes
1answer
140 views
Is it possible to calculate group 3's factor of 3 in Thistlethwaite algorithm?
https://www.jaapsch.net/puzzles/thistle.htm
I'm trying to generate 29400 ($8C4^2 * 6$) indices for each one of the cube states in G2.
$8C4^2$ = 4900 is for solving the corner and edge pieces (forming ...
4
votes
3answers
277 views
Not selling 100 pencils
A shop sells pencils only in boxes of fixed size.
It cannot sell 100 pencils.
It can sell any larger amount of pencils.
It has one of each size box on display.
question 1: What is the minimum amount ...
7
votes
1answer
226 views
A 3x3 grid with common factors
A $3 \times 3$ grid $G$ is filled with every number from the set $\{2,3,5,6,7,11,14,15,30\}$. Now a new $3 \times 3$ grid $H$ is formed, such that $H_{ij}$ is the number of neighbors of $G_{ij}$ that ...
11
votes
4answers
950 views
Deriving a 3x3 grid from another one
A $3 \times 3$ grid $G$ is filled with every number from $1$ to $9$. Now a new $3 \times 3$ grid $H$ is formed, such that $H_{ij}$ is the number of neighbors of $G_{ij}$ that are greater than $G_{ij}$....
8
votes
3answers
715 views
Perfect magic 4x4 square
Can you fill a 4x4 grid with every number from 1 to 16, such that every row, every column and every 2x2 sub-grid of numbers sum to the same value?
0
votes
1answer
82 views
Unusual 3x3 square
Can you fill a 3x3 grid with every number from 1 to 9, such that the sum of numbers in the first row is equal to the sum of numbers in every 2x2 sub-grid? Can you find multiple solutions?
8
votes
1answer
341 views
Manual tiling with 8 dodecadudes
Here are 8 dodecadudes. (Drawn from the very numerous dodecadrafters, made from 12 half equilateral triangles, dodecadudes are a subset of 770 pieces with sharp points and narrow necks excluded). ...
8
votes
5answers
1k views
Minimize 1×3 tiles on a 5×5 table to block any more 1×3 tiles
What is the minimum number of 1×3 tiles that can be put on a 5×5 table so that no more 1×3 tiles can be put on it?
Borders of a tiles are parallel to sides of the table.
It is 5 but I can not prove ...
9
votes
1answer
499 views
Plants vs Zombies!
Several plants and zombies (no more than 20 creatures in total) came to the party “Plants VS Zombies”, and it turned out that all the creatures are of different heights. When a plant speaks to a lower ...
12
votes
3answers
1k views
Swapping 3 knights in a 4x4 grid
Can you swap black and white knights in this 4x4 grid? There is one important constraint: at no point can a knight be under attack by an opponent knight. Knights move using standard chess moves (L ...
2
votes
2answers
108 views
Surrounding an L-shaped tromino
You are given an L-shaped tromino, shown below. Can you surround it by 10 more L-shaped trominoes? The new trominoes can be rotated and must touch the original tromino at an edge or a corner.
This ...
10
votes
2answers
606 views
Two football teams
Twenty two football players have agreed to split every week into two teams and play a match against each other. Every week, teams will be chosen differently, 11 players in each team, and they will ...
13
votes
2answers
920 views
Placing 9 cars into a 4x4 carpark
A carpark is arranged in a 4x4 grid and has a single entry/exit as shown in the diagram. A car has the size of a single cell of the grid. Cars can move through adjacent empty cells of the carpark ...
1
vote
1answer
265 views
Probability of a successful DT Cannon
In multiplayer Tetris, there are a number of opening setups that let you send powerful attacks very quickly. One popular opening is the DT Cannon, which allows the player to very quickly send a T-Spin ...
3
votes
1answer
184 views
Swapping eggs puzzle
You can try this with pencil and paper, or make it a physical puzzle you can try out with your kids if you have one of those large 3x6 egg cartons laying around.
Imagine you have 3 rows of 6 spots. ...
3
votes
1answer
161 views
Non-increasing arrangement
We are arranging the numbers from 1 to 8 in an order so that three consecutive terms cannot be increasing. For example, 12345678 isn’t allowed but 81436572 is. How many ways are there to do it? Please ...
2
votes
0answers
71 views
Fair and square island hopping [duplicate]
If amateur fiction is not your thing skip to the bottom.
As IP (Implausible Physics) expert for DREAM, the Department for Reckless Engineering and Advanced Megalomania you have been tasked by sheikh ...
4
votes
1answer
357 views
Two knight tours on a 4x4 grid
Two knights are placed on opposite corners of a 4x4 grid. Can you move* each knight 7 times, such that each cell of the grid is visited exactly once by exactly one of the knights?
*Note that a knight ...
1
vote
2answers
239 views
Minimize the longest King chain on a 6x6 ternary grid
This puzzle is an extension of this one: Minimize the longest King chain on a 5x5 binary board
Given a grid filled with numbers, we define a King chain to be a path on the grid such that:
The path ...
0
votes
1answer
133 views
Minimize the longest King chain on a 7x7 binary grid
This puzzle is an extension of this one: Minimize the longest King chain on a 5x5 binary board
Given a grid filled with numbers, we define a King chain to be a path on the grid such that:
The path ...