Forehead Numbers Brain Teaser

[woo2fly21]

this one was easy!!

[Honeytech11]

More of a simple way to solve this is if A B and C Are each asked what number is in your forehead then you can use that as an advantage, there were no rules for telling others number and weren't also rules for answering the wrong answer. So A says the B's number B will not get the idea why A answered that answer but C clearly sees A's and B's numbers and get's the idea. he will say A's number and then B will realise what's goin' on. That's my theory.

[HUsoldierNumber14589]

@Honeytech11 those rules are implied. If he were to make the rules that specific, they would go one for ever.

[KillerWhaleSFl]

have things to accomplish...must stop doing brain teasers

[WHITEMANDINGO]

Got it in 15 minutes.

[falqr]

I feel so proud that this is the only one I could get!

[turquoiseguy]

All of your other videos made me think a lot before I got it, and sometimes I didn't get it, but I got this one instantly.

[RisingAngelAu]

Yep...got it in a sec! Bring on some more! :)

[IVEDCtube]

The easiest, but not so interesting one... I personally thought that there will be some unique solution for all of the numbers... A little bit disappointed. :(

[kurkur078]

yup, totally agree with most of comments here, this one probably the easiest one. i got it straight away without listening to the hint. hehe. proud of myself.

what makes me sad is that this teaser does not apply to all numbers, like many of ppl had stated here.

[thesuperbouncyballs]

Let's move halloween to november 5

[SouljaBoyRealHipHop]

I figured out the solution easily, but didn't know it was right until I saw the rest of the video because I assumed that the others said "I don't know" verbatim and didn't give any information on what they thought it could be.

[GigsixstringOfficial]

I got it right!!! :)

[xsporian]

FINALLLY!!! I GOT ONE!!!

[kskthesmart]

This problem is 100% wrong... This only works for Sal since the numbers are separated by a constant value (10 in this case)

Lets assume the numbers on A is 10; B is 25 and C = A+ B = 35;

A looks up at B and C and has 2 options 10 or 60

B looks up at A and C and has 2 options 25 or 45

C looks up at A and B and has 2 options 35 or 15

None of them are conclusive... and when it is A's turn again... HE CANNOT possibly figure out that his number is 10... Any thoughts

[MayurDirector]

@kskthesmart I agree ... I tried using numbers 1, 9, and 10 and his solution does not hold.

[961kgb]

@MayurDirector lol did the exact same thing with the same numbers :P

[geistjagerin]

@kskthesmart This problem is not "100% wrong". He never said that you could deduce your number given any combination of numbers fitting the parameters laid out in the beginning. The brain teaser involves only this specific case, where you see 20 and 30.

Interestingly, you can still deduce your number correctly even if you take away the requirement that the numbers be unique. The third person would know that if he was 10, and you were 10, then the second person would have known he was 20.

[Xinfinitude]

@geistjagerin Very well said.

[pitusimx]

i found this brain teaser to be the easiest one.

[Makoroful]

@pitusimx i agree

[myname12321]

this one only works if one possible solution (in your example the 10) is half of one number you see (the 20)

does not work when you see e.g. the numbers 3 and 4

by not telling us that fact you almost made my head explode :)

[siddi117]

:O 66 likes, 6 dislikes

good video tho

[noiluigi64]

I wasnt able to solve it because i forgot the number were unique! Bah.

[D4niZeDog]

great one. the example with 20 and 30 is ok, but i wasted a lot of time trying to figure a general solution! actually, there are many cases where this has no solution, or more precisely where no one can figure out what number he has on his forehead.

[xoxoYellow]

Ppl are saying that this is the easiest one, but im still pretty juiced that i was able to figure it out :D!

Thanks KahnAcademy!!

[ocmentos]

Easiest One ;) got it without thinking abt it that much :)

[MrBuddyMaker]

Pretty easy one

[SpoiledLogic]

this one was an easy one

[vishwanathrulz]

I don't understand this. Suppose the numbers are 6,18 and 24 and I am say 6.

I see 18 and 24 and know I could be either 6 or 42, cannot conclude.

18 sees 6 and 24 and realizes it could be 18 or 30, cannot conclude.

And now 24 sees 18 and 6(me) and knows it could be 24 or 12, both of which are not repetitions of 6 or 18. So how does anyone be entirely sure, Any explanations?

[rollorhodes]

@vishwanathrulz That's why he specifically uses the numbers 20 and 30.

[merkercana]

I think this one was rather simple. I got it straight away.

[TheJuliuscanute]

Since you have mentioned A,B,C>0 how would you deduce ,when the value of A=2,B=5 and C=3?

[TheJuliuscanute]

I think A would have been able to figure out if that was the case.......hence B would be able to deduce. The strategy varies depending upon what numbers we choose.

[zhukehua]

good point : ) E-Mail him

[Sons0fMannKohral]

That is a curious case.

The unique number exclusion principle doesn't come into effect for 2,3,5

[qMaddixp]

@TheJuliuscanute same way....

[ard5608]

I like when you confuse yourself at the end ( 11:52 - 12:08 ) :p

great vid btw thanks

[dontchakno1]

i got this in aprox 1 min

[veteran25]

I got 50. Two possibilities I have: #50 or #10. If i have #50 Then Green thinks he has 20 or80 and Blue thinks he has 70 or30. If I am 10, green will be: 40or20, blue will be:30 or 10. Cant be 10 bcuz they will be the same on my head. Take the average of both of them: 80+20=100/2=50 70+30=100/2 =50. Therefore the answer is 50.

[BASS510420]

i'd just add or subtract the two numbers i see and i'd have to have one of those numbers

[iamthewalrus080391]

50 easy

[agemo2007ed]

Now, I have another problem that's a bit more difficult to solve:

the same game, only this time you see 15 and 11; 15 starts.

It goes like this:

15 you 11 (can't determine the number);

15 you 11 (still);

15 you 11 (yet still);

but after that, 15 knows the answer.

then you figure out your number and

finally 11 figures out his.

Try it, it's bit of a headache even if you know the explanation but still, you may like the challenge.

[si41tpdd1]

ok - i give up. i haven't been able to figure it out. What's the answer?

[agemo2007ed]

Excellent video that got me thinking. Especially

wonderpope's comment regarding a general solution and I think I may be close to finding one (something to do with gcd and Euclid's algorithm) but enough of that.

[rerere284]

something that confused me was that you said /inplied that it was not the diffrence.

you said the sum of A+B=C

Not minus.

very confusing to me.

[0xGEEK]

Oh man, I also thought 20 and 30 were mere examples for arbitrary numbers (except > 0, unique etc). I wasted a whole zarking week thinking about an impossible general solution...

Guess the friend of mine was right who told me 20 mins should be enough for EVERYTHING ; ) Nice job, though.

[oib4696]

I actually figured this out in about 10 seconds without looking at the solution. Yay, I'll go eat a cookie now =D

[davidmichael3d]

Well done! I got it in about a minute.

[ScythePuppet]

Nice brainteaser. I was only able to deduce that I was either 10 or 50 and was too lazy to take the extra step. But yeah.. thanks :)

[macroeconomiccrisis]

THX these are great !!!!!!!!

greetings from belgium

[wonderpope]

I didn't watch the video attentively enough and thought that the problem would have a solution in the abstract case. So I thought the guy with 20 and the other with a 30 on his forehead were only an example...I almost lost my mind with trying to find a solution to it :D

[jessicahd24]

I love the brain teasers!

[draglab2]

What program does he use for drawing?

[ekim051084]

khanacademy produces excellent videos, beyond excellent. And guess what? They're free. Take your comments elsewhere. And it's you're* not your. I believe you are (or you're) searching for the free English videos, Einstein.

[ryal214321]

no, he was right. it is your* not you're but yea I agree dsgl0p's comment was unnecessary.

[ekim051084]

OH, you are right. Wow I feel dumb. I loath people who cannot distinguish between your and you're. I was tired last night and angry at his ignorance. Thought I was correct.

[jgposner]

dsg10p - You might want to get on board or society will leave you behind.

[espen180]

I have not yet seen the slolution, but I reached the number 50 through a number of if-then statements and putting myself in the minds of the other two.

[tenshady]

THANK YOU. I guessed 10 hahahah now i understand

[dubbleplusgood]

stopped at 8:50 now,...aaaarggh of course. pretty sure I know how to figure out which one it is now. Very nice brain teaser

[dubbleplusgood]

im new at this so dont kill me for trying to be clever :)

I stopped the vid at 5:26, wondered if it's rigged in some way. To me, if all 3 heads are perfect logicians, the 1st one asked should have known A+B=C where all 3 are diff, 1 is the sum of other two and none are zero. So A>0, B>1, C>2 so if the other 2 heads are 20 and 30, his head has to be either 10 (10+20=30) or 50 (20+30=50). And unless one of them says that, how can any of them know for certain which one it is? time for the rest ...

[sjman409]

figured it out in under a minute

[ScepticalMisha]

got before the hint, yeah!

[DeifyReality]

Ez.

[ChewyWiener]

woohoo! This is the first one I got right before the hint.

Yay. I feel smart today.

Thanks for the HQ btw, it's pretty. :D

[Vyraji]

wow higher quality

[kittykattykoo]

I noticed that too! :D

[idiothek]

YESSS that was a goodie :)

[EasytoonGuy]

I paused at 5:25 (I will see the rest of the video after posting this comment to check if my answer is right) and thought about it in about 10 minutes, and my answer with explanation is (don't read any further if you're still thinking on your own): At every logician's first turn, they are all left clueless of wether their number is the product of the two others' or the other factor which together with the lowest number one of the others have makes the product, which is the highest number one...

[EasytoonGuy]

...of the others have (or in the first logician's case; either 10 (factor) or 50 (product)). BUT, at the second turn of the first logician, (s)he knows his/her number is 50, since the third logician could either have seen a 10 (in which case he would have known his number had got to be 10, though he would probably wonder why his number wasn't unique as it thereby would break the rules), or 50 (in which case he wouldn't know his number!). I hope I've got it right and haven't forgot anything here.

[EasytoonGuy]

Ha! Got it right, and without any clues besides the rules. Great brain teaser, regardless! More, more!

[Anon1696]

I THINK you forgot to add to the rules that the logicians can't lie and always tell the truth.

But hey, they might be implied since it says they are Perfect.

[Anon1696]

Awesome brain teaser, and I figured it out.

Thanks for this.

[esQo0020]

nicee

[7Truth7Society7]

when is the brainteaser playlist coming? I LOVE These things

[0800RelevantElephant]

nice!

[ImTestingSleeping]

Thanks a lot, Sal. These are always fun. Thought I had this one figured out by using a similar solution as the blue forehead one but I was glad to see it was a completely different solution.