# That Viral Math Problem (Cheryl’s Birthday) – Numberphile

### Bernard Jenkins

1. Traore Sekou

Logic is bae.

2. abde villiers

This gorilla calls himself a mathematician 😂😂😂😂😂

3. Musaic

Took way too long to explain a simple logic problem

4. Aakash Goswami

I'm 1,009,023rd viewer

5. Ronald Johnson

would i be stretching it if i said right here in this Youtube channel us plebe's are actually treated to the literal best of the best? this channel has grown on me and i've only met the narrator of this video just in the last couple of days. i don't' know why i haven't subscribed yet but putting that in now 😉

6. MichToJoshya

Why add some keywords like "gimmie" or whatever.
They didn't need to be there.

7. LukeTOK

Bernurd 😂

8. Matt Wood

May 19th

9. J L

So basically it was as if they were talking to each other in binary, exchanging "true/false" statements.

Cool.

10. Codebreaker45

3 out of 2 people struggle with fractions

11. David

S M O O C H

12. ICY 300

I guess I have no logic then 🤷🏽‍♂️ smh….

13. Ryan Soon

The Blue sharpie has a red tip

14. Yiimbob

August 17-> 1+7 =8 => August.
July 16 -> 1+6=7 => july (7th month )

15. Topher TheTenth

I thought it would be an interesting exercise to try to write this in few words. Albert isn't holding May because if he were he couldn't rule out Bernard holding 19 and already knowing everything. Similarly Albert isn't holding June because if he were he couldn't rule out Bernard holding 19 and knowing everything. So Albert holds either July or August. Once he speaks, Bernard knows that Albert holds either July or August. If Bernard were holding 14 then he couldn't differentiate between Albert holding July and Albert holding August, and Bernard wouldn't deduce the birthday at that time. But since he DOES deduce the birthday after Albert's speech, we know Bernard isn't holding 14. If Bernard is holding 15 or 17 he could rule out either July (no 15) or, respectively, June (no 17) after Albert's speech, and would know the date is (respectively) August 15 or August 17. But then Albert wouldn't be able to figure out whether Bernard held 15 and rule out July to get the date, or held 17 and ruled out June to get the date. Since Albert DOES figure it out after Bernard's speech, Bernard isn't holding 15 or 17. That means Bernard is holding 16 and Albert is holding July. Albert, holding July, knows that Bernard must be holding 14 or 16. But Albert knows that if Bernard is holding 14, Albert's speech wouldn't cause Bernard to deduce the entire birthday because Bernard would not be able to choose between July 14 and August 14, and Bernard says he CAN figure it out. So Bernard isn't holding 14 and must be holding 16. So as soon as Bernard says that Albert's speech enabled him to deduce the entire birthday when he couldn't before, it tipped of Albert to the entire birthday (July 16) as well.

16. david bishop

Thank you I love your enthusiasm and the way you teach.

17. Carlton Johnson

whose faces did you use for Albert, Bernard and Cheryl on the video?

18. Power In Knowledge

This problem really doesnt seem that complicated, the gimmes you first showed I got then I realized it was incorrect, I did a little more thinking wrote it out and got it pretty quick. Took me 20 mins tops

19. Math Man

There's a 50% chance that either Albert or Bernard shares the same birthday as Cheryl so they should just guess their own birthdays.

20. Luigi Da Leopard

Idk about you guys but that's my birthday so me and Cheryl boutta go gang out together.
Edit: where my July 16 bois at

21. איתי סמואלוב

July 16 is my sister's birthday!

22. Y Shal

Alberts initial statement is completely loaded and up to interpretation. I think there are different ways of looking at it.

For this particular solution he should have really said “I don’t know the answer and I know that Bernard could have no possibility of knowing it.” This would better imply and lead to the solution presented in this video.

Instead Albert says that he knows Bernard doesn’t know the answer. Well of course he doesn’t know the answer! He could have been whispered the 14th into his ear yet that very statement would still be valid. And this time there would be no reason to cancel out May.

23. jm jw

So, I have been able to deduce it is in July since if it was any month that had a unique day, Albert wouldn’t know that Bernard doesn’t know, and July is the only one with a non unique day, and once Albert said that, Bernard now knows the month, July, and the day, he was told, but I cannot for the life of me figure out which day it is, will finish video now, eager to find out

24. Mohamed Blaiech

but what if we include mind games. what if one of them lies, could he still beat the second guy and be the only one with the right info? and if so, what the lie should be?

25. Charis Kapas

What is funny is that based on the interpretation of Albert's first phrase "i don't know when Cheryl's……… Bernard does not know too" you can get to different results. I thought he meant "i don't know, but since Bernard does not talk he propably does not know either" and from that point , based on the rest conversation, you can conclude that the date is August 17!

26. • Kaansaticiii•

I understood how Bernard knew but I didn’t understand how Albert knew

27. General Sorrow

I figured it out, therefore did not need to watch the explanation.

28. Harsh Saraogi

Illogical too many assumption

29. Sayeth We

The Whispering was really unnecessary

30. kyarri2

im proud of myself for pauseing this opening up a spread sheet and solving it exactly how it ends up explained in the video XD

I always love watching this channel because i feel like they are talking to me lol

numberphile is like an intelligent buzzfeed

33. blackholeproductions

that's my birthday LOL

34. PETER ZARI

August 2 because that is my birthday

35. PETER ZARI

Maybe the answer is the day that the video was published

36. Shivam

I knew immediately because there was only 2 of a day and 2 of a month, and those were July, and 16.

37. ElizaBear

This turned me on working this out; awesome braingasm! So much fun! Came here to confirm my logic was sound. It was! Love it. 🙂

38. Lil Wong

Hey my birthdays in July 16th

39. Bhagwat swaroop Gupta

Why was may completely removed

40. Rajkumari Rawat

I solved it on my own!!!!!!!!!

41. Terrus Ciekawostki

I have similar riddle: There are two camels. One is purple. How much does the dog weight when it's dark?

42. James Blenkinsopp

on one hand, cool, I got the solution almost instantly, but on the other hand, I'm meant to be revising for a mechanics paper tomorrow and they're not going to be nearly as simple as this…

43. FatPodMan

My guess is July 16th

Edit: HAHAHAHAHAHAHAHAHAHAHHAHA!!!!!!!!!!!!!

44. Shreeya Mittal

That's also my birthday, although I never would've dreamed up such a screwed up way to tell people about it.

This aint it, Cheryl

46. Jesse

bernerd

47. ShaunLeeWhoo

Of course Cheryl is a Cancer..

48. FYC

Albert looks like Hitler

49. Lucas Loh

June 17

50. Archie Lambert

But Bernard could still make the statement 'I now know the birthday' if it was August 15th or August 17th?

51. Geoffrey Wood

The problem with the puzzle is Bernard can only work out the date if he has already heard Albert's second response. He doesn't have that information when he makes his deduction so he shouldn't be able to work it out.

52. Super Sophisticated

February first? Genuine guess from the beginning

Oh never mind, I didn’t know those dates listed were the actual only choices

53. WarpRulez

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.

54. ziv shayer

Happy birthday Cheryl 🙂

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

55. Tushar Goyal

and coincidentally I am watching this video on 16th July

56. agarRoyale 2002

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

57. Steel Beard

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.

58. Hackman

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…

59. MacGamer Media

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

60. GURJOT JASSAL

On which sheets they write?

61. Domihork

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

62. conner hartman

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.

63. Bilb Ono

0:23 Cheryl definitely let smash.

64. Hihkuhro

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.

65. gore buster

(1 + (1/100000000000000))^100000000000000

66. gore buster

Mr Pampena is a funny man 🤣🤣

67. Ben Hoogendyk

That was super fun

68. 5Gonza5

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

69. Kevin Salim

But… But… 16th July is my birthday.

70. ༺ڳOpernce.ฟ༻

Legit figured this out, first time I heard this,
this is easy wth.

71. Tamás Koncsos

I was wrong.

72. Bbduck Gaming

Ive randomly guessed someones birthday before, first try too.

73. Eddie Byrne

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

74. George PritsipidoulasTech

Why is may 15,16 crossed out?

75. dddiop

Simon has Keira Knightley eyes

76. Dy

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….

77. Da mamoot

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

78. tesseract `

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

79. carlosvacos 333

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

80. OmikronWeapon

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

81. 1919helio1919

5:30 I heard him say you fxxking gimme

82. Aryan Thapliyal

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

83. acxez knightNite

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

84. ezzz9

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.

86. MägiCo M

Thats was the first Numberphile video I fully understood. Yeay

87. Majd Alusta

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

88. Stephen Kenny

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

89. lagg boi

I GOT IT
Finally
Bagels are savoury donuts

90. inordirection

"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

91. Logis. exe

He predicted the death of xxx 5:12

92. Wooft

Answer*cheryl is a d*ck

93. Horrible Channel

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

94. Maximilian John

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

95. akash mishra

So which is the strongest Pokemon?

96. Raymond C.T

July 16 2021

Spiderman 3 Release Predicted On A Math Video👏👏

97. Floordford

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

98. ElPikachu

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.

99. Dan M

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

100. David Morris

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.