Brain Teaser: Blue Forehead Room

[TakeThatSociety97]

HAH nobody leaves, but everyone has a blue forehead

[palebluedot4ev]

but if no one has a blue forehead, then they'd all leave because each would see no other has B, and conclude, since at LEAST one must have B, and therefore conclude that he/she has B.

[kyle1010333]

simple everybody leaves

[thesuperqazplmkdude]

u talk too much :P, but yur AWESOME

[anayagf]

u should write the Brain Teaser somewhere... like the description....

u explainig turn a little confusing 

[westten1]

7 people had a blue forehead

[oddsolostrike]

Ha "When you REALIZE your head is blue" he tells you its blue

[AmourEtArdeur]

they could ask their fellow logicians, "is my forehead painted blue?"

[howdouwin]

As you were explaining I thought of looking into each others eye's. :(

[latetilla0205]

You didn't ask any questions?:s

[Eilavesch]

Easy, say "You guys all have blue foreheads, is mine blue too?" and go have a drink at the local pub.

[InjunJoe]

What would be the benefit of turning the lights off 100 times; no new information is gained every time the lights are turned on and off so you could turn the lights off a million times or 3 times, you still have the same information. and all 100 people get the same information at the same time so if one person realizes that he has a blue forehead then everyone will realize it.

[dRex002]

@InjunJoe ik this is a late reply, I hope you have had time to think about it. If you haven't figured it out, you sir are stupid

[InjunJoe]

@dRex002 I guess you are equally stupid dRex since you clearly don't know the answer either

[dRex002]

@InjunJoe ahhh yea I do know the answer. I got it before watching the answers video but after the hint

[InjunJoe]

@dRex002 I guess the answer has to do with these 100 guys being perfect logicians because I watched the answer and still don't buy it; if you were in a room with 99 other guys and they all had blue foreheads and you saw that after the 1st time the lights went on; why would you have to have the lights turned off and on 98 more times before you left the room?

[InjunJoe]

@dRex002 I got a better question: How do you keep an idiot in suspense??

[xandror]

The piece of information that at least 1 person has their forehead painted blue becomes useless when there is more than 2 people. If you are looking at 1 or 2 blue heads it doesn't matter, only if you saw zero blue heads would you know yours was blue. As presented no one would ever leave, he must have left an important piece of information out of the puzzle.

[leenva]

Ask your neighbor, "Do I have a blue foreheard?"

...*Door closes*...

[roophies]

You're*

[Riot454di]

Noone can know 100% if their head is blue or not, they are told at least 1 is blue and they can all see at least one is blue so that gives no further information. If anyone walks out they r a douche.

[camiller3754]

It really hurts the chances of solving when nothing was mentioned about communication restrictions.

[adam125zzz]

@camiller3754 he did

[wuzhe1997]

counting... "one person, 2 people, people.... 98 people, 99 people, oh, there are a hundred people in this room!" (quote from one of the blueheaded guy)

[SnortsOfHappiness]

All you have to do is lick your hand and slide it of your forehead where the paint is, than see if what comes off is blue.

[PiwiTeen]

i say noone leaves, because each individual will look around and see everyone's forehead painted blue and say "well it can't be me, one of them will probably get out of the room" so in the end, noone leaves cuz they all think the same thing.

[Gabumon54321]

Think, rather.*

[Gabumon54321]

Well, you can't be 100% sure, and if they're perfect logicians, they wouldn't leave. That's what I thing, at least.

[dangermous1968]

what will happen is that as soon as the lights get turned off for the 1st time they will all trample each other trying to get out of the room in the dark.........total mayham and about 20 will be seriously injured , while 6 or 7 will be stomped to death....

[dangermous1968]

@dangermous1968 hahahaha

[wadams19]

@khanacademy

can they talk to each other? can they signal each other in some way?

[paulceltics]

can't they talk?

[th33n00bi3]

this is a type of problem worth thinking



[tIs4gatorbait]

i dont understand the probelm very well

[Fatali1989]

I wish they had something on literature or philosophy. I guess that's going to be there next step.

[dadvoc666]

they'll all stay?

[MrOnairos]

can't they just tell each other they have blue foreheads?

[rotflmaopmpqxyz]

If I saw 99 other blue foreheads I think the odds that I am the one out of 100 people that doesn't have a blue forehead is much lower than the odds that these game show ppl painted all of our foreheads blue just to screw with us

[niikkeel98]

they dont

[CliffRichardsPissSac]

Frank Zappa is that you!?

[Everfalling]

man do you ever ramble. you spend 7 and a half minutes explaining a problem that would take me only 2 and you left out any details about whether or not the men in the room can talk to each other. Maybe you need a script or something? given the problem you set up all i would have to do is yell out 'hey, all of you guys have blue foreheads. is mine blue too?' and i'd be done. the number of people wouldn't matter. again, that's given the problem you set up. i don't think you set it up right though.

[Swedanyrl]

We suppose that:

- the logisists can't communicate verbally

- there is no looser, thus creating an environment where they all try to help each other

- since they can see each others foreheads(FH), they can see each others eyes.

Different scenarios:

1)one or a few of the 100 have blue FH, they'll notice that there are a few and that others are watching them.

2)majority has blue, same reasoning as above.

But what about when you see around 50? Dang...

Also, do you think that i am on to something?

[Everfalling]

@Swedanyrl the thing is he didn't say what would happen if there were less than 100 blue foreheads out of the 100 people, he said what would happen if there were just less than 100 people (of which would all still have blue foreheads)

[marsgodofwarx]

noone said they couldnt talk, so they talk and say everyone has a blue forehead.

[mbaviator]

Nobody leaves because they all think it is them who dosent have a blue forhead

[EDOM666DEVORE]

they will all look at eachother with surprise everyone they can see is blue so they think they are not and if nobody leaves and they all are like wtf noone left everyone swhould of left then they will all leave

[SpoiledLogic]

Or they can do a mind meld like Spock!

[SpoiledLogic]

they all pair up and deduce if they have a blue forehead. and hwen the lights turn off, they all get up and leave and it gets crowded at the door

[2dWorldGames]

They all die because of dehidration after 3 days

[thegoonist]

@6543Asia exactly!

[chenyimeng]

I think this video is very interesting, it makes the question looks more easier. I think the author is very smart. If next time I have a question, I will go to his website.

[1susannava]

i think that this is kind of nice of how math works in real life and how it all makes sense together. i think that there could have been a little more clear on what he was saying because u needed 2 hear what he was saying really closely. anyway other than that its really good at how he explains it

[phtmexplo]

nice riddle, i ll explain it abit :

say there is 20 ppl in the room with a blue forhead (dot). Then 80 ppl will see 20 dots, 20 ppl will see 19 dots. On the 19th time the light switches off, the ones who see 19 dots will leave (as they have a dot), the other ones who see 20 dots will wait for the 20th time the light turns off,..... but after the 19th > no dots anymore.

hope this cleared it abit for those non-understanders :)

[thegoonist]

@phtmexplo wtf? youre not even making sense

[phtmexplo]

? why not? u try explaining it then ...

[madman0004]

I am all for riddles but this one is horribly presented. Perhaps the narrator should have practiced the script at least once so that the riddle is presented in a succinct, easy-to-understand manner. It's a great riddle and I've heard variations of it before, the person telling it always has a story about where it came from: A computer science test at MIT or a job interview at Microsoft etc. etc. The core idea remains the same, however.

[mcalleja68]

just ask the person next to you if your forehead is blue

[afiq950615]

as 99 person blue..

other person will see 98 blue + me.

so if im not blue, they will stare/look at me because im different compared with others.

if they not look at me (they look at evryone)

so i know im not different so i'm blue also.

...and...

i'll leave the room

[RichyBabess]

Well everyone's going to be looking at you, due to your orange forehead being different. Everyone can see this, and deduce that no-one else has a coloured forehead other than blue and everyone else will leave the room, other than you, who sees all eyes on you. You know this wouldn't happen if you were the same as everyone else.

[djswizzair]

i love riddles =)

[Iraqii]

On the 100th light, they all leave ;)

[ZancHOS47]

and the point of this?

[bigboom273]

just rub some paint off your forehead and then look at your hand..........

[cwweric92]

this is crap

[andrija666slipknot]

u are fucking booooooring

and you are quesing too much

[yugiohsc]

this problem is like the alien abduction problem

[NinjAssassins]

hmm? what if some of them people were scared of the dark?

[e419ruth]

this is stupid. why dont they just ask the person next to them if they have a blue forehead then walk out? can't belive i just watched that, surely this isn't a brainteaser!

[kukidude70]

asshole!  Where's the solution?

[ypey1]

well guys we can all just figure this by trying out ourselves. Who's got the fingerpaint? i'll be at the lightswitch!

[tyrnazgil]

at 7:17 you say "less than 100 people"

it should be "fewer than 100 people"

[tyrnazgil]

p.s. I love your videos!

[Mronyxgiant]

no he is right he said "less than" as in <

[rorcon]

Then go put up some grammar academy videos

[olinater5]

if they are all PERFECT LOGICIANS... AND THEY KNOW THAT THEY ALL ARE... THEN IF 1 PERSON LEAVES... EVERYONE LEAVES... BECAUSE THEY ARE ALL THE SAME

[ypey1]

smart remark!

[2XTURBO]

if I see 99 people staring at me, I would guess that my forehead is not blue. If people glance at me just as they do to the other 98 people, then I would deduce that my forehead is blue and leave.

[ypey1]

hahah lol...

[yuskei3]

I've come up with two situations:

1) 1 person has a blue head. This person will see 99 non-blue foreheads and will deduce that his color is blue. He will leave at the first lights out.

2) 2 people have blue heads. These people will see 1 person with a blue forehead, so they will not leave on the first lights out. When each of them sees that the sole blue man has not left, each can deduce that the blue man can see another blue man, namely themselves. They will both leave at the second light out

[sbrawler]

Where is this reality show?

[TheBrokenSaint]

it is just a brain teaser. This is a hypothetical reality show to illustrate the brain teaser

[THeMuFFiNmAN81]

Maybe that's best for him to think. I know people who can think better if they write out the question that they're answering. Either way, he's making videos because he wants to, and they're free, so as a douchebag viewer you have no right to complain.

douchebag

[787Bisurdaddy]

You're right, I shouldn't criticize someone for making grammatical errors in an educational video, and because I did, that makes me a douchebag. I guess you've never heard of "constructive criticism."

[blackdranfire]

Your critisisum is not constructive. Avoid uses of the word waste and phrases like " crappy paint animations"

[jabmalassie]

I want more of this.

Suggest a book.

I got the first part like I think most bot not the last. You should disable comment on this.

[Kingarthur305]

true

[Kingarthur305]

Can they talk to each other?

[scorpiojudo]

Please post the solution vid as a vid responsse to this vid, I can't get back to it.

[pongman]

Lol, I'm not even going to try to solve this problem. I guess that's why it's an MIT question. My IQ is 138 and for me that's fine. I'm just glad that there are people in this world that are super intelligent for without them this planet's civilization would be at a standstill.

[jabmalassie]

I am sure my IQ is lower and you don't

to be that smart to solve this question.

It is a way of thinking.

[Xerriva]

no one leaves?

[rupeshtr]

everyone leaves

[ovaisreza]

If they are less then hundred then every body stays, because then no body else has blue forehead, he already left.

[dietsnapple89]

no one leaves because you said the producers told the logicians that only one of their foreheads will be painted blue. if you painted everyone's forehead blue then you break your own rules and the game show is Epicfail

[calvinhobbesliker2]

it AT LEAST one person painted blue.

[recordings2008]

1 vs 1blue 50% chance of red forehead 1 vs 2blues 33 % of red forhead

1 vs 3blues 25% of red forhead

1 vs 49blues 2%red forhead 1 vs 99blues 1 % red forhead

for the logicians the 1 percent probability is good enought to make them walk away

_with hint_

[gamemaster014]

i mean it's very likely that your forehead is painted blue since your looking at 99 other blue foreheaded people but your not exactly that yours is blue it's extremely likely that it is but you don't know so i don't think anybody would leave the room

[IKilled007]

What happens is that you realize that your head is painted blue. You see 99 blues and don't know what yours is at first, but you know everyone else sees 98 blues + yours. But they're really seeing 99 blues. If they don't stand up it means the only forehead they are unsure of is their own. That means your head is blue. But they all come to the same conclusion about themselves. Everyone leaves.

[oodava]

That's what I came up with also.

[gamemaster014]

i don't think anybody would leave the room

[Slicksz]

I'm guessing that they'd all leave. I have no clue why I think that, but I do lol. Time to watch the solution ;].