I’m so happy I’m here to be able to get to talk about a viral maths problem. I mean when does that happen? Come on. And this is not in the maths community. We’re talking about like in the big wide world. This is really exciting. Have you heard of it, Brady? It’s got a few names, at the moment. It’s like the Singapore maths problem it’s a competition maths problem originally from Singapore. it’s also “The Cheryl problem” Not saying anything against Cheryl Cheryl’s not a problem Cheryls are usually pretty nice So if you’ve been living under a rock I’ll quickly explain what it is It’s actually, from a mathematical point of view it’s a classic logic problem It involves 3 people Cheryl, Albert, and Bernard Ok So Cheryl is trying to get Albert and Bernard to guess her birthday For what reason, we don’t know Maybe she’s just trying to play Albert against Bernard Or maybe Albert and Bernard, y’know, are trying to scam Cheryl for like her secret question for a bank account Who knows? But Anyway She gives them a bunch of options Ok so, Cheryl has shared a possible pool of dates But now, what’s she’s done is she’s actually gonna give a piece of information to Albert and a piece of information to Bernard An incomplete piece of information about her birthday so this is how it goes Albert, I’m going to tell you the month, – whisper whisper whisper and Bernard, I’m going to tell you the day, – whisper whisper whisper So now all they know is Bernard knows that Albert knows the month and Albert knows that Bernard knows the day But they don’t know it together And so then, they have a little exchange So Albert states I don’t know when Cheryl’s birthday is But I know that Bernard does not know too then Berard replies at first I didn’t know when Cheyrl’s birthday is, but I know now. So now suddenly, Bernard knows But what happens? Is Albert left the in dark? No Somehow, Albert is then able to deduce then I also know when Cheryl’s birthday is. So now they both know. And presumably, Cheryl is either happy or upset Or her bank account has been broken into. Something like that. We don’t know exactly. The question is how can you find out the information Nothing is explicit, is it Brady? it just seems like they playing games Are they Jedis? What’s going on? Are they reading each other’s minds? and this is the nature of these logic problems Believe it or not, this isn’t the first time I’ve been faced with a problem like this As mathematicians, we get asked these problems all the time And so the way to answer this Is to actually break it down and try and attack it logically because otherwise you’re not going to get anywhere. I am now going to rearrange this information here Because this is really important we need to have something to focus on So what I’m going to do is I’m actually going to split out the dates at the top here I’m going to..I’m going to put 14 uh… 15 The dates there in columns and in rows I’m going to place the months So here is May So now, what I’m going to do there is I’m just going to have a look at this so May 15 A cross is just basically, this is an option. This could be it. 16 or 19 Yeah, these are also contenders. What I’m going to do is actually give us a bit of understanding here because once we get into the logic we could get lost Let’s look at the options for this column. For the day or the date 14 how many different options do we have? We’ve got 2 ok 15 we have 2 16 we have 2 17 we have 2 18 we have 1 and 19 we have 1 as well. Ok, now let’s have a look for months. So now for months how many options do we have for each month? So we have 1,2,3 for May 2 for June 2 for July and 3 for August See where I’m going with this, Brady? So now what’s going on here is that I’ve actually just, I’ve just spread out the information Before I start bringing in the weird conversation they had The weird terse conversation that they had so now Albert says I don’t know, I don’t know what the birthday is. OK. (Brady: Of course he doesn’t.) Yeah, but that’s a weird thing to say. Can I just say that? It is a weird thing to say because in a logic problem Obviously you don’t. We both know the game here. You’ve only been told months. There is 3 options, 2 options, 2 options, 3 options. The only way you’d know is if there was 1 month there with only 1 date. Just saying that. I’m just saying that. What’s interesting though is Albert says “But I also know that Bernard doesn’t know.” We’ve got the information in a way that, ya know, I’m gonna be able to work with, but just looking at this straight away there are some red-hot dates. Here June 18 Well if you have a look at it it’s only one 18 that applies to any of the months It happens to be June. In actual fact the same for May there’s 19 19’s not anywhere else Can you see that, Brady? There’s nothing around there What this means is if Cheryl went over and said to Bernard “18” or “19” *smooch* What that means is straight away Bernard would be in. It’s like a gimme. It’s like “Bang”. I know exactly what it is. You give me the information. It can only be 1 thing. But see, Albert in sort of a gloating way, I mean really Albert needs a bit of emotional intelligence I think because he’s basically gloating and he’s going, “

## 100 Comments

Watching this video again after 4 years, I must say that I find it a bit annoying to watch.

No offense intended for Mr. Pampena, but that presentation was a bit annoying. A bit too much repetition, a bit too much of saying the same things again and again, and with needless detail, the actual whispering was unnecessary and a bit annoying, and the whole thing just takes too much time to get to the point. I think this could have well been explained, clearly and understandably, in 5 minutes.

Happy birthday Cheryl 🙂

Great Video- watched it today for the 3rd time. (last time about 3 years ago)

and coincidentally I am watching this video on 16th July

11:06 in column 14 there's an "o" not a "0"

When I read the problem, I understood it as meaning that Albert knew that Bertrand didn't know, not from logic, but from observation. After Cheryl gives each man a piece of the answer, Albert and Bertrand watch each other and then Albert says that he doesn't know, and Bertrand doesn't either. That eliminates only June 18 and May 19. But then Bertrand, seeing that Albert doesn't know, realizes that June 17 is off the table as well, since if Albert had been told June he'd have immediately known the full date. Since Cheryl told Bertrand "17" then he knows now that August 17 must be correct, since it is the only other date with a 17.

It took me like, 2 minutes to solve it and i am still in school. Its not all that hard tbh… Its just the wordings that get most ppl, me included. At first i thought it was a joke but then somebody told me it was actually possible, and once i knew it was possible, it was a cake walk…

8:54 Ok just worked it out, Cheryl’s birthday is July 16.

On which sheets they write?

Why does the guy sound like he's talking to an idiot?

Bernard never indicates that albert would know now, so in albert's head, it could have been either 15, 16 or 17. If it was 15, bernard would have known but it could have also been the 17th. I dont see how albert could have known just from the statement that bernard made.

0:23 Cheryl definitely let smash.

Lol i just used a little logic. The wording was bs so i just counted the words. Theoreticly saying that Bernard made a long pause in his sentence the total words up to the comma are 7. And he knows the month so it has to be July. And Alberts total words are 16 so its the 16th. And because they are mathmaticians and as we all know they count every word they both know the answer.

(1 + (1/100000000000000))^100000000000000

Mr Pampena is a funny man ??

That was super fun

I will be honest I absolutely didn’t understand a thing…

But… But… 16th July is my birthday.

Legit figured this out, first time I heard this,

this is easy wth.

I was wrong.

Ive randomly guessed someones birthday before, first try too.

Sorry dude , staring at you from under that proverbial rock, as I don't use social media much , but looking forward to your explanation .

Why is may 15,16 crossed out?

Simon has Keira Knightley eyes

Without watching the video from 2:30 onward, i did get to a point where i had only these (3) as options.

– July 16

– Aug 15

– Aug 17.

when Bernerd says "at first i didnt know but NOW i do" …., that is applicable for any of those (3) options. I couldn't figure out, from that point, how could Albert know Bernard has 16???

I'm lost in Simon's explanation on the final clue …. , how can albert "just know" it has to be july"??? how can Albert be 100% sure that Bernard wasn't given '15' or '17'?

Let's assume Albert was given 'July'. Then since Benard said 'Now i know' …, then this WOULD BE the ansewr.

But let's assume Albert was given 'August' and , yes, he would've had ambiguity between '15' and '17'. from Bernard's perspective, he knows for sure the birthday since he has the date. But from Albert's perspective, i don't see how he can know for sure what Bernard has (btwn 15 and 17) IF HE WAS GIVEN AUGUST….

i think it's july 16th, went through it myself after they showed the table

I am at 2:08, and I say it is the 16th of July

I dont understand why he erased 15 an 16 may. Pls explain

Afterwards Cheryl goes: "wow, you guys are so smart. that killed a bit of bit of time waiting for my quarterback boyfriend to pick me up and take me out for my birthday"

Albert & Bernard: FFFFFFFUUUUUUUUUUUU

5:30 I heard him say you fxxking gimme

The best part about this video is that it was published on my birthday that is 17 April!!!!!! Wow!!!!!

Why could it not be August 15th or 17th?

It's a cute problem but most times it fails.

By logic, August 17, because if you don’t want to guess, don’t choose option that has one possible answer.

Thats was the first Numberphile video I fully understood. Yeay

I would have had a foursome with them and knew everybody’s birthday leaving my own unknown!!!

I love the number five videos and the 60 symbols and the rest that you do

I GOT IT

Finally

Bagels are savoury donuts

"If he's holding on to 15, 16, or 17 in his head–or really, in his heart, because we're dealing with cheryl here" loool

He predicted the death of xxx 5:12

Answer*cheryl is a d*ck

The problem should’ve said Cheryl tells him it’s on July and it’s also on the 16th

Isn t there a failure in the question. I think Bernhard should say at last That he know now the Date and Albert does know it now too. Because Bernhard Could have the 15th, 16th or the 17th and the last sentence he Said doesnt give Albert the informations he needed. Sorry for my bad englisch

So which is the strongest Pokemon?

July 16 2021

Spiderman 3 Release Predicted On A Math Video??

Your logic is flawed. Go back to the beginning and act as though Cheryl said one of the days that you crossed out.

Ok i promise i calculated it myself, my estimation is july 16, because albert knows for fact bernard doesnt know when cheryl birthday is, we can remove from the ecuation the months june and may, as those have the days 18 and 19, respectively, wich are unique and would let bernard deduce the date. The dates we are left with are july 16, aug 15 and aug 17. months with day 14 cant be as they are present both in july and aug, wich are completely possible. however albert responds he knows the date too. so, whats the only month that has 1 unique correct day left? july, july 16. Hope it is correct.

Omg i didnt know russell brand was on this channel!!

I dont understand why may got thrown out. Just because it had a day that was a gimmie doesnt mean it couldnt have been the 15th or 16th of may.

Why he cuts the whole month of May and june ?

Nobody knows, everyone pretends !

4 years late, but july 16th is my birthday!

i gave this a go before he explained it it and got i right! it was messy but it worked

We pay people to do this all day

I was actually able to solve this!! Big brains!

2019 and I'm still laughing about : "19 (kiss)" lol

Hey cool, my birthday is also July 16.

Great video. Thank you so much.

great professor

this is a logic problem, not math.

I didn't understand it only because the word gimme what is that

July 16

How did this become a math problem?

FUC%ING YOUTUBE APP PAUSE!!!

Idiots at YouTube decided the pause should darken the image and cover it with writing so you can't see the image.

So, let's pause to see the problem… NOPE AS$HOLES AT YOUTUBE MADE THAT IMPOSSIBLE!!!

I still did not understand why he took july and may out

what if albert is blufing?

I am unreasonably proud of myself for solving a four year old logic problem.

How tf did I get this in 30 sec.

gimme

I only got it right because i guessed my own birthday

You are totally wrong, the right answer is august 17.

Albert: what's your birthday

Cheryl: whisper whisper whisper

This is so fun, it reminds me of sudoku. Also, my birthday is July 16 as well!

Man…Thats my birthday

At about the 7 minute mark, can someone explain how he was able to also get rid of the dates in May?

Idk I did this in like 40 seconds

i'v got aug 17 teen lol

The clock on the fireplace shows 7-16 🙂

I did it in my head before he had a chance to explain it. 😀

EDIT: Help, I'm too exited to stop smiling and it hurts.

easy

A: I know B doesn't know because there are 2 options in my month.

B: If A knows I don't know, then it can't be May or June. There is now only one month to my date. I know the birthday.

A: Now I know the birthday, since there is only one date for July.

At the end is it wrong because bernard knew because he was told the date so only he knows what date it was but the 3 options for albert cant be taken apart because he has the option of 3 dates he impossibly could know

but why cant it be any date in August except for the 14th? he could exclude may and june now it isn't the 14th because in that case a wouldn't know for sure but is it was August 15 then B would know because he could exclude may and june and there is only one 15 left. but why does A know? He cant assume it is July because B has 3 gimmies!!!!

@5:24 that was so weird ?

game theory!

The answer is my birthday?

Sorry, but if Albert can have the 15th, 16th or 17th, how can Bernard know he has the 16th?

Reminds me a bit of sudoku

But they never told eachother date or month it doesn’t make sense

3:30 July 16th.

Only with the knowledge that Albert by some magic knows the date by the end of the problem does the logic hold up. How would Albert know it was in July and not August? Just because there is only 1 option in July doesn't logically prove it must be in July right? If you follow the same logic but assume the date was August 17th then Albert would have been wrong. So we have to assume that Albert, by some magic, knows that it's in July and only then can we assume that it must be the only month with 1 option left. I don't like that lol

This was such as delightful little problem!

WOW

I’d say FCK Cheryl and Move on ….

Nice, I figured out the solution to this one

A fantastic logic question; and I really don't know why I made a silly mistake while solving it; but yeah, I did it all by myself.

Very basic game theory problem…

Mate once you put it in a matrix I saw the answer… Maybe that's the aspergers talking

Is it just me, or was this solution immediately obvious?

My birthday's June 18th lol

My birthday!! 🙂

Or just stalk cheryl on facebook and find it