Your Favorite Brain Teaser

[Latinidiot]

There is a car crash. The father is killed and the son is rushed to hospital. The boy is taken directly into surgery but the surgeon says

"I can't operate on him, he is my son."

How can this be?

his mother is the surgeon

 

[ciancon]

http://blogs.ft.com/undercover/2010/03/the-hardest-logic-puzzle-ever/

Hey everyone! Post your favorite brain teaser. Also check back and try to answer other people's.
Here's mine:
You are on a journey to the village of truth. You come to a fork in the road with a sign that says village of truth and village of lie but no arrows to designate which way leads to which village. A man stands at the side of the road. He says that you can ask one question of him and from that answer you can try to figure out which way to go. However, you don't know which village the man is from. What do you ask him to get you to the village of truth?

You'd like this one. It's a development of your brainteaser and i've been trying to figure it out for days!

http://blogs.ft.com/undercover/2010/03/the-hardest-logic-puzzle-ever/

 

[Latinidiot]

*Dons Riddler suit*
Hello Batman! Can you solve this riddle?

The maker doesn't want it, the buyer doesn't need it, and the user never sees it.

What is it?

*twirls cane*

A coffin.

So I just got ninja'd, but yeah a coffin.

I thought it was a donor organ. Curse my too complicated mind!

 

[Latinidiot]

A verse from the bible?

Seriously though, I have no idea.

I present as a counter riddle -
I have two U.S. coins that add up to fifty-five cents. One is not a nickel. What coins are they?

Nope, sorry Batman.

Is your riddle two quarters?

Two quarters would only add up to 50c, so no.

a nickle nd 50 cent. one isn't a nicle, but theother one is.

 

[JoJo Bizzaro 7]

Time for his sons to get a job? I have no clue XD

I kind of fail at these, but I have one:

A woman is reading in her house by the sea. She gets tired, turns off all the lights, and goes to bed. The next morning, she opens the paper to read "2000 killed in boat crash." She then immediately jumps out the window and kills herself. Why?

I assume that she lives in the Lighthouse and they were killed because she turned off the light?

Bing bing bing!! Way to go!

Here's a tougher one:

A girl's mother dies, and at the funeral, she meets a nice young man. A week goes by... then the girl stabs her sister to death. Why?

Before you freak out, this is a question I was posed when I was in high school. One answer means you have some dangerous tendencies.

She killed her sister to meet the guy again. Brutal stuff.

There is a house with all it's sides pointing south. It is an old Victorian house with a white picket fence.
There is a bear outside. What color is the bear?

 

[stubbmann]

Time for his sons to get a job? I have no clue XD

I kind of fail at these, but I have one:

A woman is reading in her house by the sea. She gets tired, turns off all the lights, and goes to bed. The next morning, she opens the paper to read "2000 killed in boat crash." She then immediately jumps out the window and kills herself. Why?

I assume that she lives in the Lighthouse and they were killed because she turned off the light?

Bing bing bing!! Way to go!

Here's a tougher one:

A girl's mother dies, and at the funeral, she meets a nice young man. A week goes by... then the girl stabs her sister to death. Why?

Before you freak out, this is a question I was posed when I was in high school. One answer means you have some dangerous tendencies.

She killed her sister to meet the guy again. Brutal stuff.

There is a house with all it's sides pointing south. It is an old Victorian house with a white picket fence.
There is a bear outside. What color is the bear?

White, the house is at the north pole.

 

[2xDouble]

CURSE YOU BATMAN!!!

Umm....Try and solve this, Batman!

I have eyes but cannot see, ears but cannot hear, and nerves but cannot feel.

What Am I?

Terry Schiavo.

My riddle: You and 99 other people are being taken to some evil mastermind's lair, where each of you will be put in a line and have either a black hat or a white hat placed on your head. In the line, each of you can see all the people in front of you, but not any of the people behind you. The mastermind will start at the back of the line and go through the line asking each person for their hat color. Everyone who answers correctly will be freed; everyone who answers incorrectly will be shot. What's your best strategy?

Ask the guy behind you what color the hat is?

And mine was supposed to be a corpse, but I guess that works too.

I should expand a bit: you're not allowed to talk after the hats are placed on your heads, but you can hear what the people behind you say and whether they were right.

Oh, and I'm aware that I'm a terrible person.

Look at the hat as it's being put on your head. duh.
OR Answer correctly that you do not know the color of your hat. (although that will probably get you killed for being a smart-ass)
OR Look carefully at the line ahead of you and try to discern a pattern of distribution, while paying close attention to the answers behind you to confirm.
OR (Smart-ass answer #2): shake your head violently to remove the hat from your head, then answer correctly "I'm not wearing a hat". (you could modify this plan to get the hat into your hands, observe its color, then replace it)

You can see the hats of people in front of you. And the people behind you CAN communicate with you (you can overhear their answer to the nutjob), it's just the only information they can give is White or Black. The guy at the back names the colour which he sees an even number of, say, black. He has a 50/50 chances of dying. The guy in front counts the hats in front of him. If he sees an odd number of blacks, he must be black. The next person counts, if he sees an even number of blacks, he must be black. And so on. 99 people guaranteed to live, 50/50 chance of all 100 living.

That's a good answer, but it assumes the white and black hats are evenly distributed, giving a true 50/50 chance (or that they alternate in a logical way). Just because you happen to count more black hats than white doesn't increase your chances of answering correctly. what if there are 99 black hats and you happen to have the only white one?

 

[2xDouble]

He could keep moving instead of stopping to monitor the turtles progress

He's not stopping. The turtle's progress is being monitored.

By not running only half the distance every time?

You're getting close, but he's not actually stopping, he's accelerating the whole time. He doesn't have to run half the distance each time, but that's what's being monitored. And remember, the turtle is still moving forward in that time.

The sum of an infinite series can be finite.

True, but this man isn't limited by the distance he can cover, he's merely accelerating at each exponential interval. It's his ACCELERATION that is accelerating, not his velocity. (that's the second integeral, for you Calculus students). all he has to do is keep running faster than 1cm/minute and he will overtake it.

Also, considering an average man's step is 1 meter, roughly, and the turtle is moving at a constant rate of 1 cm/min, the man has already overtaken the turtle with a single step. Given that conclusion, the distance between the man and the turtle is INCREASING, therefore the "half the distance" acceleration somewhat balances out to a regular clip. Perspective is so much easier than integrals...

 

[TotallyFake]

CURSE YOU BATMAN!!!

Umm....Try and solve this, Batman!

I have eyes but cannot see, ears but cannot hear, and nerves but cannot feel.

What Am I?

Terry Schiavo.

My riddle: You and 99 other people are being taken to some evil mastermind's lair, where each of you will be put in a line and have either a black hat or a white hat placed on your head. In the line, each of you can see all the people in front of you, but not any of the people behind you. The mastermind will start at the back of the line and go through the line asking each person for their hat color. Everyone who answers correctly will be freed; everyone who answers incorrectly will be shot. What's your best strategy?

Ask the guy behind you what color the hat is?

And mine was supposed to be a corpse, but I guess that works too.

I should expand a bit: you're not allowed to talk after the hats are placed on your heads, but you can hear what the people behind you say and whether they were right.

Oh, and I'm aware that I'm a terrible person.

Look at the hat as it's being put on your head. duh.
OR Answer correctly that you do not know the color of your hat. (although that will probably get you killed for being a smart-ass)
OR Look carefully at the line ahead of you and try to discern a pattern of distribution, while paying close attention to the answers behind you to confirm.
OR (Smart-ass answer #2): shake your head violently to remove the hat from your head, then answer correctly "I'm not wearing a hat". (you could modify this plan to get the hat into your hands, observe its color, then replace it)

You can see the hats of people in front of you. And the people behind you CAN communicate with you (you can overhear their answer to the nutjob), it's just the only information they can give is White or Black. The guy at the back names the colour which he sees an even number of, say, black. He has a 50/50 chances of dying. The guy in front counts the hats in front of him. If he sees an odd number of blacks, he must be black. The next person counts, if he sees an even number of blacks, he must be black. And so on. 99 people guaranteed to live, 50/50 chance of all 100 living.

That's a good answer, but it assumes the white and black hats are evenly distributed, giving a true 50/50 chance (or that they alternate in a logical way). Just because you happen to count more black hats than white doesn't increase your chances of answering correctly. what if there are 99 black hats and you happen to have the only white one?

That's why the guy at the back (the first guy to guess) has a 50/50 chance of death. Once he announces his answer the rest of the queue KNOW the distribution of hats. It's even as in an even number, not an even distribution.
I'll explain again.
The guy at the back sees 70 black hats and 29 white hats. He calls out black, telling everyone that there is an even number of black hats. He happens to have a black hat as well, and lives.
Next guy counts, sees 70 black hats. 70 is even, so he has to be black.
Next guy counts, sees 69 black hats. As 69 is odd he MUST be black so that the first guy saw even black hats.

 

[2xDouble]

That's why the guy at the back (the first guy to guess) has a 50/50 chance of death. Once he announces his answer the rest of the queue KNOW the distribution of hats. It's even as in an even number, not an even distribution.
I'll explain again.
The guy at the back sees 70 black hats and 29 white hats. He calls out black, telling everyone that there is an even number of black hats. He happens to have a black hat as well, and lives.
Next guy counts, sees 70 black hats. 70 is even, so he has to be black.
Next guy counts, sees 69 black hats. As 69 is odd he MUST be black so that the first guy saw even black hats.

Right. Now look at the pattern: everyone says black until someone dies, then everyone says white until someone dies. Does that really give you a higher chance of survival? This also assumes the entire line is aware of and understands your plan, and that you are able to communicate your idea to the entire line before the hats are placed, but after the villain announces the plan. That's already discounted because the original post stated the hats were already on you before the psycho explains them. Even a single panicked answer could destroy your plan, or everyone could be killed.
The only way is to play the odds. Assume that there is some logic to the distribution of white and black hats, and count how many of each is ahead of you. If there are more white than black, you can conclude that your hat is black, inferring that there are more white hats ahead of you than behind you. Then, you listen and track the responses of the people behind you. If they also show more white hats than black, you can then assume that your hat is more likely to be white, because there are more white hats than black altogether. That is the only logical solution I can think of that doesn't require cooperation and coordination of the entire line... or cheating. (see my previous posts)

However, a completely illogical solution would be to incite a riot and attack your captors. Even automatic weapons have limited effectiveness in close quarters, and they will not be able to kill all of you. Depending on how organized you are, or how panicked you can get them, you odds will be likely higher than 50/50. In addition, you'll have the satisfaction of knowing the madman won't be doing it again.

 

[TotallyFake]

Right. Now look at the pattern: everyone says black until someone dies, then everyone says white until someone dies. Does that really give you a higher chance of survival? This also assumes the entire line is aware of and understands your plan, and that you are able to communicate your idea to the entire line before the hats are placed, but after the villain announces the plan. That's already discounted because the original post stated the hats were already on you before the psycho explains them. Even a single panicked answer could destroy your plan, or everyone could be killed.
The only way is to play the odds. Assume that there is some logic to the distribution of white and black hats, and count how many of each is ahead of you. If there are more white than black, you can conclude that your hat is black, inferring that there are more white hats ahead of you than behind you. Then, you listen and track the responses of the people behind you. If they also show more white hats than black, you can then assume that your hat is more likely to be white, because there are more white hats than black altogether. That is the only logical solution I can think of that doesn't require cooperation and coordination of the entire line... or cheating. (see my previous posts)

However, a completely illogical solution would be to incite a riot and attack your captors. Even automatic weapons have limited effectiveness in close quarters, and they will not be able to kill all of you. Depending on how organized you are, or how panicked you can get them, you odds will be likely higher than 50/50. In addition, you'll have the satisfaction of knowing the madman won't be doing it again.

There's no chance to discuss? That's the entire point of the exercise, otherwise you can only guess randomly as you have NO reason to suspect a pattern and there's no guarantee that there will be one.
If you CAN discuss the plan then this method is a perfect as you can get. 99 guaranteed, 1 50/50 chance.

 

[2xDouble]

Here's a riddle I heard, but sadly never found the solution to:

Seven white horses
On a red hill,
Now they are dancing
Now they are still

What is it?

the answer is not a carousel, that's all I know.

 

[2xDouble]

There's no chance to discuss? That's the entire point of the exercise, otherwise you can only guess randomly as you have NO reason to suspect a pattern and there's no guarantee that there will be one.
If you CAN discuss the plan then this method is a perfect as you can get. 99 guaranteed, 1 50/50 chance.

No, there is no chance for discussion, because:

I should expand a bit: you're not allowed to talk after the hats are placed on your heads, but you can hear what the people behind you say and whether they were right.

Oh, and I'm aware that I'm a terrible person.

Again, you're not looking at the entire pattern... simply because there is an odd or even number of either color ahead of you has no effect on which color your hat is. and it's NOT 99% guaranteed. It is all still 100 50/50's. your pattern, if you could coordinate it somehow, might raise the overall odds of survival but does nothing for your individual odds except give you confidence in your guess. What happens if you're the first white hat when everyone behind you says "black"? you die. Try it with 10 of your friends (you play the madman, but please don't actually kill them) you'll see what I'm talking about. It's logical probability, and logic has to start with an assumption somewhere.

 

[2xDouble]

Here's a riddle I heard, but sadly never found the solution to:

Seven white horses
On a red hill,
Now they are dancing
Now they are still

What is it?

the answer is not a carousel, that's all I know.

Teeth, precious? Yes, precious; teeth. Hehe.

works for me.

 

[TotallyFake]

There's no chance to discuss? That's the entire point of the exercise, otherwise you can only guess randomly as you have NO reason to suspect a pattern and there's no guarantee that there will be one.
If you CAN discuss the plan then this method is a perfect as you can get. 99 guaranteed, 1 50/50 chance.

No, there is no chance for discussion, because:

I should expand a bit: you're not allowed to talk after the hats are placed on your heads, but you can hear what the people behind you say and whether they were right.

Oh, and I'm aware that I'm a terrible person.

Again, you're not looking at the entire pattern... simply because there is an odd or even number of either color ahead of you has no effect on which color your hat is. and it's NOT 99% guaranteed. It is all still 100 50/50's. your pattern, if you could coordinate it somehow, might raise the overall odds of survival but does nothing for your individual odds except give you confidence in your guess. What happens if you're the first white hat when everyone behind you says "black"? you die. Try it with 10 of your friends (you play the madman, but please don't actually kill them) you'll see what I'm talking about.

Okay, i assumed there was time for discussion BEFORE the hats were put on. It's how I've heard this puzzle before and is the ONLY way you can get more than 50/50 odds. Whether or not there's an odd or even number of hats in front of you WILL allow you to determine you hat colour IF you know whether or not there's an odd or even number of hats in front of the person behind (ie. how does YOUR hat alter the odd/even hats). Want me to go through it again. Lets just have 10 people. Heck, make it 9 black, 1 white.
Back guy (black) sees 8 blacks. Blacks are even so he says black to signal that there are even blacks (remember, this only works if this signal has been agreed before the hats are place. And this WILL have been agreed before because otherwise the puzzle is worthless). He happens to be black, so he lives.

Next guy, black. Sees 7 black hats. From the signal he knows there should be an even number of hats. So he knows he must be black.
Next guy. The signal said even blacks. Two blacks have gone. Therefore still even blacks. He sees 6 black hats. Therefore white.

If the signal is agreed before hand, all but one person is guaranteed to live. If you want to disprove me show me a situation step-by-step in which this method fails. There isn't one.

 

[2xDouble]

Okay, i assumed there was time for discussion BEFORE the hats were put on. It's how I've heard this puzzle before and is the ONLY way you can get more than 50/50 odds. Whether or not there's an odd or even number of hats in front of you WILL allow you to determine you hat colour IF you know whether or not there's an odd or even number of hats in front of the person behind (ie. how does YOUR hat alter the odd/even hats). Want me to go through it again. Lets just have 10 people. Heck, make it 9 black, 1 white.
Back guy (black) sees 8 blacks. Blacks are even so he says black to signal that there are even blacks (remember, this only works if this signal has been agreed before the hats are place. And this WILL have been agreed before because otherwise the puzzle is worthless). He happens to be black, so he lives.

Next guy, black. Sees 7 black hats. From the signal he knows there should be an even number of hats. So he knows he must be black.
Next guy. The signal said even blacks. Two blacks have gone. Therefore still even blacks. He sees 6 black hats. Therefore white.

If the signal is agreed before hand, all but one person is guaranteed to live. If you want to disprove me show me a situation step-by-step in which this method fails. There isn't one.

I'll give you 3
1) someone in the middle of the line forgets the signal or pattern and answers incorrectly
2) someone in the line miscounts the number of hats he or she sees, throwing off the signals.
3) the villain decides to kill people randomly regardless of whether the answers are correct (seriously, trusting a villain to behave honorably after kidnapping you and forcing you into a game for your life?)

 

[TazTheTerrible]

My fav riddle:
The billiard balls. There's 12 of them and one is either slightly lighter than the others or slightly heavier. You have a set of scales (ones that strike one way or the other, not indicate weight) and you get to weigh three times. By the end you have to be able to determine which ball is the odd one out and whether it is heavier or lighter.

Obviously the difference is too small for you to tell by guessing manually.

1st weigh in: 6 balls on one scale - 6 balls on the other, whichever bunch are heaviest keep those 6 for the next weigh in
2nd weigh in: 3 balls on one scale - 3 balls on the other, whichever bunch are heaviest keep those 3 for the next weigh in
3rd weigh in: 1 ball on each scale, keep the third on the table, scales will tell you which is the heavier ball, if both balls are the same than the one left on the table must be the heaviest.

Not correct, the odd one out can also be lighter, in which case this doesn't work.