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Discussion Starter · #1 · (Edited)
When is Cheryl's birthday? The math problem gone viral - CNN.com

ok, I don't understand why people are having so much trouble with this question. It's pretty obvious right? Oh wait, yeah, it's worded pretty horribly. But still the answer shouldn't be too difficult to find.


to check your answer:
 
July 16



with explanation:

 
knowing that the other person doesn't know the birthday means it eliminates months where the day is unique and gives away the answer.

The fact that Bernard knows now means it couldn't have been 14 because the previous information would not have narrowed it to a definite day, so is either 15, 16, or 17

Now that Albert knows Bernard knows, the only way this could contribute to him knowing the exact day now from before knowing only the month is with the throwing out of the 14 and having it leave only one possible answer.
 

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I tried to do it yesterday. I read it trough about ten times without understanding; then after reading the explanation I realized I had skipped one sentence every time (the part about how she told the month and the day to the boys). So typical. I was really tired, but I still don't understand how I manage to do that kind of a thing so often.

(My knitting projects look horrible because I accidentally skip some parts even though I try to be so careful.)
 

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My mind just sorta went over and over and I still couldn't get it, I knew it was the same pattern and thing used in einstein's puzzles which I usually solve pretty quickly, but I really couldn't see this one through. Maybe I'm just tired these days. :p
 

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Albert and Bernard just become friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates. Albert and Bernard are no longer planning to buy Cheryl a birthday present. Fuck you, Cheryl... it was a simple question, no need to be a bitch about it.
 

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It would've been alot easier to solve if it would've been clearer who's saying what to whom though... That english, wtf. ;D
 

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The phrasing annoyed me enough to not bother with the problem at all. I still think it is a logic problem, not a math one.
 

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Actually. I don't think there is a way to get the answer:

Albert and Bernard just met Cheryl. “When’s your birthday?” Albert asked Cheryl.

Cheryl thought a second and said, “I’m not going to tell you, but I’ll give you some clues.” She wrote down a list of 10 dates:

May 15, May 16, May 19

June 17, June 18

July 14, July 16

August 14, August 15, August 17

“My birthday is one of these,” she said.

Then Cheryl whispered in Albert’s ear the month — and only the month — of her birthday. To Bernard, she whispered the day, and only the day.

“Can you figure it out now?” she asked Albert.

Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either.

So we know that Albert either got August or July. This is the only way Albert could know with certainty that Bernard does not know. As June and May have unique days (and Bernard received a day).

Bernard: I didn’t know originally, but now I do.

Bernard now knows that Albert knows that Bernard does not know for sure. So he knows that Albert got August or July. Bernard says he knows. So this means he did not receive a 14. He has 15, 16 or 17. Now he knows as they are all unique

Albert: Well, now I know, too!

This is where I am confused. How does Albert know if it is August 16, August 17 or July 15?
 

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Discussion Starter · #12 ·
This is where I am confused. How does Albert know if it is August 16, August 17 or July 15?
Well first, it's July 16. But anyway Albert can only know for sure if it was between the 14 thrown out and one other choice. Remember Albert knows the months. If it was either aug 16 or 17 he wouldn't know, so by the fact that he does know, it's July 16.
 

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Actually. I don't think there is a way to get the answer:

Albert and Bernard just met Cheryl. “When’s your birthday?” Albert asked Cheryl.

Cheryl thought a second and said, “I’m not going to tell you, but I’ll give you some clues.” She wrote down a list of 10 dates:

May 15, May 16, May 19

June 17, June 18

July 14, July 16

August 14, August 15, August 17

“My birthday is one of these,” she said.

Then Cheryl whispered in Albert’s ear the month — and only the month — of her birthday. To Bernard, she whispered the day, and only the day.

“Can you figure it out now?” she asked Albert.

Albert: I don’t know when your birthday is, but I know Bernard doesn’t know, either.

So we know that Albert either got August or July. This is the only way Albert could know with certainty that Bernard does not know. As June and May have unique days (and Bernard received a day).

Bernard: I didn’t know originally, but now I do.

Bernard now knows that Albert knows that Bernard does not know for sure. So he knows that Albert got August or July. Bernard says he knows. So this means he did not receive a 14. He has 15, 16 or 17. Now he knows as they are all unique

Albert: Well, now I know, too!

This is where I am confused. How does Albert know if it is August 15, August 17 or July 16?
It's July 16th because Albert has been told the month, and he said that he knows her date of birth. He wouldn't have known if it would be August as there are two possible answers. By elimination it must be July 16th.
 
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Well first, it's July 16. But anyway Albert can only know for sure if it was between the 14 thrown out and one other choice. Remember Albert knows the months. If it was either aug 16 or 17 he wouldn't know, so by the fact that he does know, it's July 16.
Yeah haha. Figured it out on the way home from work. Long car rides ftw! The only better vehicle for thinking is a shower.
 

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Strange that it has "gone viral" despite being fairly straightforward (and also as much a maths problem as a sudoku is, which is to say not really one at all). I only found out about it earlier today, by which point it was already described as having gone viral - so have I just missed it, or is the virality of this problem being somewhat overstated?
 

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I question the virality of this 'viral' math question. I didn't see it trending on twitter even once.

The answer: Cheryl is a bitch. She's weirder than those guys who buy 50 watermelons.
 

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I'm just gonna straight up say it:

I'm damn good at maths, I've done up to the equivalent of Calculus IV (that's how it's measured in the US, right?) and have plenty more to come. I got a First in my exams, too.

I have no idea how to answer this question. It's not maths, it logic, and it's written in bad English so there. HUMPH. :happy:
 
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