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A puzzle based on combinatorics, which is the study of counting discrete structures. Use with [mathematics]
4
votes
How many descendants can this spaceship crew produce?
I think they can, in theory,
In the following way.
Second generation
Now we have a second generation consisting of
Third generation
Now we have a third generation consisting of
Four …
40
votes
Two chessmasters at work
After 40 moves by each player, 150 minutes have elapsed on each player's clock. So the average time taken for a move is exactly $150/40=3.75$ minutes, i.e. 3 minutes and 45 seconds.
Question 1
The a …
7
votes
Accepted
Coffee machine queue
Let's call someone a Halfer if they have a €0.50 coin and a Wholer if they have a €1.00 coin. The problem is essentially to find the number of possible queues such that
since the machine takes a €0 …
14
votes
Accepted
99 Bags of Apples and Oranges
Let $B_1,B_2,\dots,B_{99}$ be the bags, in increasing order of number of apples contained; say the number of apples in $B_i$ is $a_i$ for all $i$.
If our 50 bags grabbed are $B_{99}$, one of $B_{98}$ …
14
votes
Accepted
The Stubborn Tenant
The puzzle is
Proof
Split the infinite grid up into NW-SE diagonals of length 1, 2, 3, 4, ... Assign each cell a value
Then the total value of all occupied cells is an invariant, because each …
12
votes
How many ways can you find the word DIAMOND in this diamond?
The answer is
the proof being as follows. (Thanks to @El-Guest for finding the error in my previous reasoning!)
This assumes that rotations and reflections of the same path count as different fr …
5
votes
Accepted
Attacking queens revisited
Yes: the solution was already mentioned in this answer, sourced to this page.
9
votes
Knights attacking exactly three knights
First observation: the colour of the knights corresponds to the colour of the squares - the black knights must be all on one colour and the white knights all on the other colour, since a knight's move …
5
votes
Accepted
Attacking queens
Here's the solution:
It's interesting to note that the domination problem has very few solutions for 3 queens on a 6x6 chessboard as compared to other possibilities. (Neither of those links contain …
3
votes
Pedro's pawn game
Solution to the question as stated
It can be done in 92 moves. Here's how (using standard chessboard notation, rows numbered from a to g and columns from 1 to 7):
move the white pawn on a1 to a5 (4 …
5
votes
Place 4x12 detainees on a 7x7 grid of cells
Inspired by my answer to a more general puzzle.
This is a stream-of-consciousness answer showing exactly the thought process needed to find the solution. If you just want to know the answer, skip to …
6
votes
Accepted
Colouring an n times n grid using n colours
It's possible if and only if
Proof is as follows.
$n\leq3$
$n>3$
For example, with $n=7$ we have
1
vote
A Two-pan Non-Equal Arm Scale
Partial answer
Unless my physics is wrong, the fact that the pans have different weights and the arms have different lengths means that there exist constants $a$ and $b$ such that the two pans balanc …
3
votes
Finding numbers having exactly two distinct digits
Fix $K$. We will find conditions on $i$ for the road sign numbered $i$ to be as desired.
Write $i$ as a $K$-digit number, say $i=i_1i_2i_3\dots i_K$ where each $i_k$ may be any digit from zero to nin …
5
votes
Accepted
Coloring an n by n grid with four colors
$n$ even
A solution is possible for
Proof is as follows.
$n$ odd
A solution is possible for
Proof is as follows.
How did I find this? (SPOILERS)
Well, let's start with $n=5$. Now we kno …