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

## 100 thoughts to “That Viral Math Problem (Cheryl’s Birthday) – Numberphile”

1. Inquisitour M29 says:

AC

2. kalyani hota says:

Ans is whisper whisper whisper

3. snooz mr.snooz says:

Smooch

4. Jeffery Thompson says:

This isn’t hard. And I’m 13

5. Ali Butt says:

Sorry man I lost track

6. Harry says:

This was pretty easy

7. Rinila Chatterjee says:

The only mathematical thing I need to know is how many Sharpies Numberphile has

8. Poperpoppop 1 says:

August 15th is my birthday

I got July 16th

Haven't watched the solution yet

Basically, because Albert knows that Bernard doesn't know, that means that it's not a unique day. This eliminates May and June because they both have a unique day (a day that only appears once in the choices). 19 is unique in May and 18 is unique in June.

So all that's left is July and August.

Bernard says he knows the answer, which means that it's a day that's in either August but not July, or in July but not August. Because of this, we can remove July 14 and August 14.

Now, July 16th is left, August 15th is left, and August 17th is left.
Since Bernard knows the day, he knows the date.

Now, Albert knows the answer even though there are three options left.
If the month he was given was August, he wouldn't know the answer yet because it could be August 15 or August 17. HOWEVER, he says he knows the answer! That means that the month isn't August. All that's left is July 16.

🙂

10. Matthew Ramos says:

Why couldn’t Albert just say the month and Bernard the date and they both know

11. Broman Clusterfield says:

I watched this video because I live under a rock.

12. TheGerkuman says:

So they're not gloating. They've just found a way to share the info with each other without Cheryl working it out. Subterfuge.

13. G. Dalazen M. says:

This is really cool, but I like it especially because my birthday is July 16 🙂

14. Ivan Pecotić says:

This is to EZ

15. Dan Aviv says:

tbh I just paused the video when the problem was presented and solved it.

But they got to know the answer without writing anything…

When is cheryls birthday

18. T Phas says:

Could anyone explain me why Albert can cut May and June out. I can really use some helps here. Thanks.

19. NikiWin says:

999,584 views

999,999 views!

21. Mrinmoy sen says:

So much for a birthday, I will skip that one , THANK YOU!!

22. Elijah Senju says:

The big white world

23. Traore Sekou says:

Logic is bae.

24. abde villiers says:

This gorilla calls himself a mathematician 😂😂😂😂😂

25. Musaic says:

Took way too long to explain a simple logic problem

26. Aakash Goswami says:

I'm 1,009,023rd viewer

27. Ronald Johnson says:

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 😉

28. MichToJoshya says:

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

29. LukeTOK says:

Bernurd 😂

30. Matt Wood says:

May 19th

31. J L says:

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

Cool.

32. Codebreaker45 says:

3 out of 2 people struggle with fractions

33. David says:

S M O O C H

34. ICY 300 says:

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

35. Ryan Soon says:

The Blue sharpie has a red tip

36. Yiimbob says:

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

37. Topher TheTenth says:

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.

38. david bishop says:

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

39. Carlton Johnson says:

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

40. Power In Knowledge says:

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

41. Math Man says:

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

42. Luigi Da Leopard says:

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

43. איתי סמואלוב says:

July 16 is my sister's birthday!

44. Y Shal says:

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.

45. jm jw says:

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

46. Mohamed Blaiech says:

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?

47. Charis Kapas says:

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!

48. • Kaansaticiii• says:

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

49. General Sorrow says:

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

50. Harsh Saraogi says:

Illogical too many assumption

51. Sayeth We says:

The Whispering was really unnecessary

52. kyarri2 says:

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

55. blackholeproductions says:

that's my birthday LOL

56. PETER ZARI says:

August 2 because that is my birthday

57. PETER ZARI says:

Maybe the answer is the day that the video was published

58. Shivam says:

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

59. Elizabelle says:

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

60. Lil Wong says:

Hey my birthdays in July 16th

61. Bhagwat swaroop Gupta says:

Why was may completely removed

62. invisibly visible says:

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

63. Terrus Ciekawostki says:

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

64. James Blenkinsopp says:

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…

65. FatPodMan says:

My guess is July 16th

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

66. Shreeya Mittal says:

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

68. Jesse says:

bernerd

69. ShaunLeeWhoo says:

Of course Cheryl is a Cancer..

70. FYC says:

Albert looks like Hitler

71. Lucas Loh says:

June 17

72. Archie Lambert says:

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

73. Geoffrey Wood says:

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.

74. Super Sophisticated says:

February first? Genuine guess from the beginning

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

75. WarpRulez says:

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.

76. ziv shayer says:

Happy birthday Cheryl 🙂

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

77. Tushar Goyal says:

and coincidentally I am watching this video on 16th July

78. agarRoyale 2002 says:

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

79. Steel Beard says:

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.

80. Hackman says:

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…

81. MacGamer Media says:

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

82. GURJOT JASSAL says:

On which sheets they write?

83. Domihork says:

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

84. conner hartman says:

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.

85. Bilb Ono says:

0:23 Cheryl definitely let smash.

86. Hihkuhro says:

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.

87. gore buster says:

(1 + (1/100000000000000))^100000000000000

88. gore buster says:

Mr Pampena is a funny man 🤣🤣

89. Ben Hoogendyk says:

That was super fun

90. 5Gonza5 says:

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

91. Kevin Salim says:

But… But… 16th July is my birthday.

92. ༺ڳOpernce.ฟ༻ says:

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

93. Tamás Koncsos says:

I was wrong.

94. Bbduck Gaming says:

Ive randomly guessed someones birthday before, first try too.

95. Eddie Byrne says:

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

96. George PritsipidoulasTech says:

Why is may 15,16 crossed out?

97. dddiop says:

Simon has Keira Knightley eyes

98. Dy says:

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….

99. Da mamoot says:

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

100. tesseract ` says:

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