232
votes
183
votes
Accepted
160
votes
122
votes
Is this Hashi puzzle unsolvable?
Here is another argument for why this puzzle is unsolvable.
The Handshaking Lemma is the simple but powerful idea that each edge uses up a value of 2 (one from each end), so the total amount must be a ...
- 50.8k
112
votes
Accepted
101
votes
93
votes
Accepted
90
votes
Three mathematicians are forever in Prison
When the mathematicians are asked for their answers, each answers "light mode" if their light is currently on, or "dark mode" if it is currently off.
- 1,041
88
votes
Quick!!! You got only 30 seconds
No real-world car can accelerate faster than $5g$, so the maximum acceleration of the car is under $50m/s^2$. Even if the car instantly decelerates (against a wall or something), the fact that $P$ can ...
- 2,684
86
votes
Accepted
Three logicians walk out of the bar
If the first were sober s/he would answer yes. So the first is drunk.
If the second were sober s/he would answer yes. So the second is drunk.
If the third were sober s/he would answer yes. The ...
- 22.8k
83
votes
What is my four digit car number?
Last digit is the double of first digit;
Middle two digits are same. It has to be a number when multiplied by 2 has the last digit in it;
Last two digits are the double of first two digits
The only ...
- 1,129
83
votes
Accepted
Thanks, dad! Never again
I think that the next number will be
Because you are watching
The numbers are called out by
Addendum
- 133k
82
votes
Accepted
71
votes
71
votes
Accepted
Guessing hat colors. 4 prisoners
This is a bit of a stretch and I'm not sure it'd even work, but...
- 1,488
67
votes
Accepted
67
votes
How can I convince my colour blind friend that his two balls have different colours?
You can for example photograph the balls in two separate photos and use a program Color Picker (like this one) showing your friend that the RGB color codes are different.
- 1,320
66
votes
Accepted
66
votes
Accepted
65
votes
Crack the lock code
You can see from clue 4 that there are no 7, 3, or 8.
You can see from clue 5 that there is a 0 in either the first or second slot, and from clue 3 that it must be in the first slot. That means the ...
- 145k
64
votes
Accepted
Haters gonna hate
The greatest number of students that could be summoned is
Proof you cannot summon more:
Here is a situation where the principle can summon that many:
- 32.1k
63
votes
Accepted
63
votes
Accepted
This is it. This is the one. Find your wife
This is a comprehensive and complete round up of the answers submitted and will credit the people who solved it.
Clues from the flight schedule (thanks to seasnake, MPeti and Dan Russell):
And:
63
votes
Accepted
100 pieces 1 opportunity, choose wisely!
What you're missing here is the chance of playing at all, given that the game ends when someone finds the prize. (or, chance of finding a prize goes to 0, which is the same thing)
...
- 7,734
61
votes
Accepted
58
votes
How can I convince my colour blind friend that his two balls have different colours?
Show him the balls through
Edit: He might agree that there is some difference that he can't see, but he's still not convinced that it's the colour. Then
- 581
56
votes
Accepted
55
votes
Accepted
Is this chromatic puzzle always solvable?
Yes, it's possible.
Start with the various 2x2 squares.
Ignoring symmetries of rotation and color swapping, there are 1 combination of 1 color, 3 combinations of 2 colors, and 2 combinations of 3 ...
- 7,438
52
votes
Accepted
Oh dang, I have to change my whole answer!
One answer is
To find this, I first noted that each of the blanks must be at least 8, since there are already 8 digits different from the given one. None of the blanks can be 8, though, since that ...
- 676
52
votes
Accepted
Tiling with T-tetrominos in gravity
TLDR: I'll fill the board and prove that the solution is unique.
First, let's start by:
I'll paint those green:
Let's repeat those steps a few more times, using orange, blue, red and purple, in ...
Only top scored, non community-wiki answers of a minimum length are eligible