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04/21/2015 07:42:49 AM · #1 |
Albert and Bernard just met Cheryl. âWhenâs your birthday?â Albert asked Cheryl.
Cheryl thought a second and said, âIâm not going to tell you, but Iâll give you some clues.â 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
âMy birthday is one of these,â she said.
Then Cheryl whispered in Albertâs ear the month â and only the month â of her birthday. To Bernard, she whispered the day, and only the day.
âCan you figure it out now?â she asked Albert.
Albert: I donât know when your birthday is, but I know Bernard doesnât know, either.
Bernard: I didnât know originally, but now I do.
Albert: Well, now I know, too!
When is Cherylâs birthday? |
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04/21/2015 08:03:57 AM · #2 |
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04/21/2015 08:29:40 AM · #3 |
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04/21/2015 08:29:50 AM · #4 |
| We had this at work the other day and I know the answer so I'll refrain, although there is contestatio of a possible second option for the answer. |
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04/21/2015 09:03:54 AM · #5 |
Albert and Bernard went out for drinks and to the strip club and decided that it was not worth knowing Cheryl's birthday.
I came up with 2 different answers
hahah!
Message edited by author 2015-04-21 09:08:31. |
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04/21/2015 09:04:28 AM · #6 |
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04/21/2015 09:13:47 AM · #7 |
| that's a really strange conversation to have with someone you've just met |
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04/21/2015 11:45:07 AM · #8 |
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04/21/2015 01:25:01 PM · #9 |
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04/21/2015 01:26:19 PM · #10 |
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04/21/2015 01:44:46 PM · #11 |
| The dress is black and blue. |
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04/21/2015 05:31:03 PM · #12 |
Cheryl Cole: June 30
Cheryl Hines: Sept. 21
Cheryl James: March 28
Cheryl Ladd: July 12
Cheryl Strayed: Sept. 17
Cheryl Tiegs: Sept. 25
My cousin Cheryl: Oct. 9
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04/21/2015 07:17:42 PM · #13 |
| our Russian cousin, Cheryl Nob, 26 April. |
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04/21/2015 07:22:06 PM · #14 |
Originally posted by tnun:
our Russian cousin, Cheryl Nob, 26 April. |
I thought she was Ukrainian. |
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04/21/2015 07:27:47 PM · #15 |
Originally posted by bvy:
Cheryl Strayed: Sept. 17
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Cheryl strayed on Sept 17th...Does her husband know? |
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04/21/2015 07:45:14 PM · #16 |
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04/21/2015 08:13:08 PM · #17 |
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04/21/2015 08:17:17 PM · #18 |
Originally posted by PennyStreet:
Ginger Rogers? |
Dream date! (Except that she's dead.) |
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04/21/2015 08:44:50 PM · #19 |
Originally posted by bohemka:
Originally posted by tnun:
our Russian cousin, Cheryl Nob, 26 April. |
I thought she was Ukrainian. |
I think there is some sort of war about that. |
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04/21/2015 10:29:42 PM · #20 |
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04/22/2015 03:52:48 AM · #21 |
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04/22/2015 03:53:13 AM · #22 |
not stationary
Message edited by author 2015-04-22 03:53:35. |
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04/22/2015 06:36:26 AM · #23 |
Originally posted by Giles:
We had this at work the other day and I know the answer so I'll refrain, although there is contestatio of a possible second option for the answer. |
I don't think there is, actually.
I'm not sure if Albert could actually have known after Bernhard's statement, but assuming that Albert's last statement is correct, we can know.
Tricky question, though.
Message edited by author 2015-04-22 06:48:26. |
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04/22/2015 09:02:29 AM · #24 |
Warning, there be answers ahead. With logic.
Looking at this from an "extrapolate the data" view, if one knows only the month and the other only the day, the first thing you have to do is rule out any unique numbers, so you scrub June 18 and May 19 from the equation.
May 15, May 16
June 17
July 14, July 16
August 14, August 15, August 17
With this, some might say that June 17 fits the bill since it's the only option left that can be interpolated as a set unto itself, which both people could then come forth with knowing. That's going to get you some traction with groups of people, but it isn't correct.
Since the first statement can be seen as a bit of a taunt of "I know and you don't :p", you have to rule out June since it would be so obvious to both and initially it is stated that they do not both know until the second person knows.
You now have May 15/16, July 14/16, August 14/15/17.
Since Bernard knows what month it is, the logical conclusion is August 17 since it is the only unique date left which fits the bill for Albert knowing the date and Bernard knowing the month, after ruling out June as a unique datum in a set.
Many people still are up in the air on this not accepting this as the logical and correct conclusion, and having valid workable theories to prove why their date is correct.
As detailed by a mathematician how there can be two correct answers:
Originally posted by "smart math guy":
If Bernard is so confident, he must have a unique date. We know itâs not 18 or 19. What other unique date can it be? There are two 14s, two 15s, two 16s and two 17s - but Bernard has eliminated June 17 - leaving him with August 17 only. Thatâs how he worked it out.
Albert is furious Bernard beat him to the answer. Albert puts himself in Bernardâs shoes, running through the six steps above. Finally Albert reaches the same conclusion we have, Bernard must have 17. Albert announces he knows the answer too.
So August 17 is a valid answer.
But how is it possible to get two different answers to the same question?
It is all about how you interpret the first statement. If Albert has to deduce that Bernard doesnât know, then we get July 16.
But if Albert knows that Bernard doesnât know â in other words, that this is a statement of fact, rather than a deduction â then we get August 17.
This incredibly subtle change â deduction vs fact - completely changes the nature of the question. Indeed, with fact interpretation the reader can now deduce the answer from just the first two statements of the conversation, whereas the argument for July 16 does require all three statements.
full text located here
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Message edited by author 2015-04-22 09:13:02. |
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04/22/2015 09:13:38 AM · #25 |
| It's a trick question. Before Bernard and Albert have even uttered a syllable they've deduced that Cheryl is high maintenance and they're lying about knowing her birthday just so they can get to the pub sooner. |
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