Newest Questions
29,037 questions
73
votes
7
answers
143k
views
Twelve balls and a scale
You are given twelve identical-looking balls and a two-sided scale. One of the balls is of a different weight, although you don't know whether it's lighter or heavier. How can you use just three ...
user88
9
votes
1
answer
648
views
Moving a pawn around on a chessboard
Suppose you have a checkerboard with two opposite corner squares removed, like this:
Can you move a pawn on the board only horizontally and vertically, one square at a time, and have it touch every ...
user88
15
votes
1
answer
4k
views
Burning ropes as timers - How many time intervals can be measured?
Note: This is a follow-up question to Burning ropes as timers. The following question and its answers may contain spoilers.
To sum up the puzzle, these are the rules (slightly modified):
You have ...
- 3,503
48
votes
6
answers
8k
views
Burning ropes as timers
You have 2 identical ropes which burn at a specific rate, and an unlimited supply of matches. When you light one end of a rope, the fire will take exactly 1 hour to travel to the other end of the rope....
- 3,503
15
votes
9
answers
105k
views
Flip edge in Equator layer
If you use the Beginners' Method to solve the Rubik's cube, and solve the equator layer (the layer between the U-layer and D-layer), it can happen that one of the edges is flipped, like this:
...
- 495
11
votes
2
answers
3k
views
How many minimal-clue sudoku puzzles are there?
It is known that there are a minimum of 17 clues needed to create a proper sudoku puzzle.
For how many puzzles is 17 sufficient? From the history of the minimal proof, I gather that there are ...
- 2,800
8
votes
1
answer
977
views
What is the fewest number of clues on a rotationally symmetrical sudoku grid with a unique solution?
It is known that the minimum number of clues a sudoku must have to have a unique solution is 17. But on every website I've seen for them, I haven't found any that are rotationally symmetrical.
I once ...
user88
9
votes
5
answers
4k
views
A rope-cutting problem
Here's another oldie from a book, slightly paraphrased to make it more quantitative.
You're in a room with a ceiling exactly 100 feet high. Two thin but sturdy ropes are hanging from fixed hooks at ...
user88
9
votes
1
answer
1k
views
How to solve 5-10 letter anagrams (without a computer or electronics)?
How can I solve 5-10 letter anagrams without a computer or electronics?
I was playing the Word Scramble category in the game Quiz Up and found it complicated, only in some parts. At home, I used an ...
- 338
3
votes
2
answers
2k
views
Are there any topology puzzles similar to The Seven Bridges of Königsberg?
Leonhard Euler proved that it is impossible to solve this puzzle:
The challenge is to take a walk around the area depicted, and cross each bridge (yellow) exactly once. Using topology, Euler proved ...
- 667
13
votes
2
answers
11k
views
Removing numbers from a full sudoku puzzle to create one with a unique solution
What is a simple and efficient way of removing numbers from a full sudoku puzzle to create one with a single unique solution?
I have already figured out how to generate sudoku grids, like this one:
<...
- 4,677
13
votes
5
answers
1k
views
How do you create a general locked-box problem?
In this question, I presented a puzzle involving seven locks, with a Grand Vizier and four slaves.
But I would like to use this style of puzzle in some of my own secret-unlocking riddles, in order to ...
user88
17
votes
4
answers
49k
views
Strategy to solve the Missionaries and Cannibals problem
In the Missionaries and Cannibals problem:
Three missionaries and three cannibals must cross a river using a boat which can carry at most two people, under the constraint that, for both banks and ...
- 1,289
3
votes
3
answers
1k
views
What is a good notation for trial-and-error when solving Sudoku on paper?
When solving Sudoku on paper, I sometimes get to the point where I need to resort to guessing or bifurcation. At that point, I might guess one number, and continue solving, changing that guess if I ...
user121
40
votes
3
answers
6k
views
Pirates and gold coins
A group of N pirates has come by a chest containing 200 gold coins. Their rules require that the coins be distributed by the following approach. The pirates are ranked from fiercest to meekest and ...
- 9,004