The birthday puzzle.

[Heptagon_ru]

Maybe some of you haven't seen this logical maths problem. It seems impossible to solve, but actually it's not too hard to find the answer. I have already enjoyed massive holywar among my colleagues, so can't resist to share the puzzle with you. :)

Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates.

May 15 16 19
June 17 18
July 14 16
August 14 15 17

Cheryl then tells Albert and Bernard separately the month and the day of her birthday, respectively.

Albert: I don't know when Cheryl's birthday is, but I know that Bernard does not know, too.
Bernard: At first, I didn't know when Cheryl's birthday is, but I know now.
Albert: Then I also know when Cheryl's birthday is.

When is Cheryl's birthday?

[InsaneJ]

Logic puzzles hurt my brain :(

Answer....
It's July 16th.

[Heptagon_ru]

Wow, that was quick. :) Oh, cmon, put the answer under spoiler at least :))))))

If it hurts, it is still present, and it's great. :)))

At work we (after 3 days) still can't convince one colleague about the answer and about the logic to get it. Every day it gets very emotional :)))

[LegendOfAir]

Well I officially fail at these things. I can't figure it out, even though J already said the answer, I have no idea how it go to that answer xD

[Heptagon_ru]

:) One of the explanations:
http://www.theguardian.com/science/alexs-adventures-in-numberland/2015/apr/13/how-to-solve-albert-bernard-and-cheryls-birthday-maths-problem
(http://www.theguardian.com/science/alexs-adventures-in-numberland/2015/apr/13/how-to-solve-albert-bernard-and-cheryls-birthday-maths-problem)
For a small hint you can read only the beginning of the explanation.

Personally I'm bad too, and in this case I even didn't have a chance to solve it, because the initial post where I read that was styled more like a joke, so I thought it was a joke :( And only after reading a comment with the explanation .... :( Well, since it was also a bit hard to get the idea from the explanation, at least some profit for the hurtless matter inside my head :)))

[InsaneJ]

The solution lies in the order in which Albert gains more information from his conversation with Bernard.

What does he know already? What knowledge does he gain by Bernard first not knowing, and then knowing because Albert didn't know? Because Albert didn't know, one scenario was eliminated for Bernard, which one? What does that leave him with?

[galros]

Maybe some of you haven't seen this logical maths problem. It seems impossible to solve, but actually it's not too hard to find the answer. I have already enjoyed massive holywar among my colleagues, so can't resist to share the puzzle with you. :)

Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates.

May 15 16 19
June 17 18
July 14 16
August 14 15 17

Cheryl then tells Albert and Bernard separately the month and the day of her birthday, respectively.

Albert: I don't know when Cheryl's birthday is, but I know that Bernard does not know, too.
Bernard: At first, I didn't know when Cheryl's birthday is, but I know now.
Albert: Then I also know when Cheryl's birthday is.

When is Cheryl's birthday?Just over a year later, but still entertaining ;) Thank you from me and my friends. It was really funny Tea break on a Friday afternoon. We've solved it without cheating. If someone has not seen it, try to solve it ;) Good luck!

[latyper]

I got as far as narrowing it down to

July 16th
August 15th
August 17th

Even after reading the explanation I don't get is how does Albert know that the 16th is the right date. Why shouldn't Albert suspect that Bernard was told the day was the 15th or the 17th? If Bernard was told by Cheryl, "my birthday is on the 15th (or the 17th for that matter)" everything Bernard said would have been the same. Lets assume Bernard was told it was on the 15th.

Bernard: At first, I didn't know when Cheryl's birthday is, but I know now.

At first Bernard didn't know when Cheryl's birthday because all he knew was it was on the 15th. He initially knew that the birthday could be on May 15th or August 15th. With the information Albert provided, Bernard can rule out May and June. Bernard could still work out that her birthday was on the 15th. His statement, "At first, I didn't know when Cheryl's birthday is, but I know now"would be exactly the same. The same holds true if Cheryl told him her birthday was on the 15th.

If Bernard's statement equally supports her birthday being on the July 16th, August 15th or August 17th then how does Albert work out Cheryl's birthday?

EDIT:

I re-read the explanation. I forgot that Albert knew the month all along. He knows from the very start that her birthday is in July. He knew her birthday was either on July 14th or July 16th before anyone said anything. When Bernard said "I know now" he was telling Albert the information Albert needed to rule out July 14th and workout the answer.