Most Popular
1500 questions
33
votes
3 answers
A Strangely Familiar Probability Problem
You're taking the final exam for your Introduction to Probability and Statistics course, and once again you've failed to study (you were up all night solving online brainteasers).
The exam is multiple choice. It asks 120 questions, each of which has…
COTO
- 10,083
- 36
- 91
33
votes
2 answers
The Time Machine works but my friend is gone!
So my friend Fred and I have been developing a time machine. I know it sounds impossible but we think we've hit a breakthrough! Fred has had a successful test run but he didn't tell me where he was going! Right before he left he sent me this weird…
Engineer Toast
- 17,506
- 1
- 49
- 152
33
votes
1 answer
Professor Halfbrain and the sum of the digits of all divisors
Yesterday I met professor Halfbrain in the city. The professor looked tired and somewhat exhausted. He told me that he had spent his nights and days with adding up digits of divisors of positive integers. For instance the integer $n=12$ has the six…
Gamow
- 45,573
- 10
- 147
- 381
33
votes
1 answer
The mystery flag
Part two - The mystery flag - twisted
Based on these flags:
(Count the sun in the Argentina flag as yellow)
Which country does this flag belong to?
Please explain why
EDIT: I added a black background to clarify what is flag and what is…
Beastly Gerbil
- 58,036
- 8
- 166
- 314
33
votes
3 answers
The Nuclear Missile Standoff
Three world powers have just entered a nuclear war!
They have been waiting for this day for decades; everyone has their nuclear missiles ready to fire, and only need to choose a target.
The Chinese missiles hit their target 30% of the time.
The…
IQAndreas
- 714
- 6
- 20
33
votes
5 answers
Slicing a rectangle
A friend presented this nice little puzzle to me yesterday. You're given a rectangle which is dissected by one of its diagonals, as well as another line that only meets one of the two remaining corners.
The smallest triangle's area is $2$ and the…
Martin Ender
- 1,601
- 13
- 23
33
votes
1 answer
Pan. Zoom. ENHANCE! ...hmmm
Agent: OK, let's test this out... pan over to that art on the wall... zoom in... Enhance!
Agent: [under breath] I've always wanted to say that...
Agent: Excellent, it's crystal clea-- Wait... What's that reflection?
Me: It looks like a…
Alconja
- 37,250
- 16
- 116
- 164
33
votes
1 answer
A little abbreviated response
This is a short riddle which has a short answer.
I look like I have no memory,
but I am fine,
but turn me around
and I may knock you down.
Tom
- 70,468
- 4
- 249
- 361
33
votes
2 answers
A riddle I made up for my kids a while ago
Explain "Two has three, and four has three, but three has only one".
David Goldfarb
- 421
- 4
- 7
33
votes
1 answer
What am I? I always play fair
What am I?
A word of 6 letters,
I always play fair
Minus my first I am cold
Minus my last and jumbled,
I'll give you a massage
Jumble my whole,
You'll learn nothing from me
I always play fair
Hint to those still trying to solve…
Beastly Gerbil
- 58,036
- 8
- 166
- 314
33
votes
12 answers
Unexpected hanging paradox
The unexpected hanging paradox is as follows:
A judge tells a condemned prisoner that he will be hanged at noon on
one weekday in the following week but that the execution will be a
surprise to the prisoner. He will not know the day of the…
Neil
- 2,116
- 2
- 18
- 26
33
votes
18 answers
Draw a line through all doors
I saw the following problem on 4chan and couldn't solve it:
It's very likely to be some kind of troll (no solution).
I'm hoping to see some rigorous proofs that disprove the existence of such a line.
Gabriel Romon
- 1,079
- 1
- 13
- 19
33
votes
3 answers
Maximum time for ants to fall off stick
Suppose there are $n$ ants on a stick which has length 1. At any time, the ants may be facing left or right, the initial directions of the ants are arbitrary. Each ant can be modeled as a point in the stick.
At time 0, all ants start moving in the…
justhalf
- 6,014
- 2
- 31
- 49
33
votes
12 answers
Knights, Knaves and Normals
In the kingdom of Boolistan, every inhabitant is either a Knight, Knave or Normal. Knights can only make true statements, Knaves can only lie, and Normals must either tell the truth or lie.
Warmup: The local tavern only allows Normals (no one can…
Mike Earnest
- 32,336
- 6
- 92
- 237
33
votes
4 answers
Doctor's Dilemma
I found the following puzzle by Martin Gardner in an archived copy of Isaac Asimov's Science Fiction Magazine. I've heard it before, but in a more vulgar setting, involving condoms instead of gloves. I think mr. Gardner knew this version too, hence…
SQB
- 5,118
- 31
- 58