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Maths problem







Moshe Gariani

Well-known member
Mar 10, 2005
12,068
16th July

May and June ruled out first.
Then it can't be one of the two 14s.
And it can't be one of the two remaining August dates.
 








Moshe Gariani

Well-known member
Mar 10, 2005
12,068
Can't be June 17
 


Bad Ash

Unregistered User
Jul 18, 2003
1,900
Housewares
16th July.

Can't be 18th or 19th.
Can't be May or June.
Can't be the 14th.
That leaves 2 dates in August, so must be the one remaining date in July, 16th July.
 










Diablo

Well-known member
NSC Patreon
Sep 22, 2014
4,173
lewes
Stolen shamelessly from the Guardian...

see here

I reckon it's 17th August. Happy to share working but let's see what others can come up with first. Any takers?




Can`t be May or june as if 18th or 19th bernard would know straightaway...Can`t be 14th as there are two options as can`t be August as again there are two options so yes it has to be for them both to know 16th jul.
 




Simster

"the man's an arse"
Jul 7, 2003
54,110
Surrey
Okay I've changed my mind. Now I think it's 16th July. Good tester, although it's much more about logic than maths.

Yeah, 16th July.

I read your original answer, then got to July 16th, then spent 10 minutes scratching my head as to why I was presumably wrong. :lolol:
 




SweatyMexican

Well-known member
Mar 31, 2013
4,099
My birthday, July 16th.

The first line rules out May and June, becuase there is one date in each that doesn't have a pair, and therefore Albert can say that Bernard will not know, eliminating those two months

The second line (in my lines of logic) rules out July 14th, and August 14th, becuase Albert eliminates the first 2 months, meaning the number Bernard has must be in either June or May, becuase Bernard has options eliminated by June and May's elimination. This means the date has to be a 15,16,17 or 18, which cannot be a 14, becuase there is no 14 in May or June.

And the last line rules out the 2 August dates, becuase Bernard says he knows what it is, and therefore it must be the last date in July, becuase otherwise Bernard couldn't know the answer if there were two dates left, which there are in August.
 
Last edited:




Phat Baz 68

Get a ****ing life mate !
Apr 16, 2011
5,023
I think its the 16th July
 


Biscuit

Native Creative
Jul 8, 2003
22,205
Brighton
The first statement excludes that the birthday is in a month with a unique day number (May 19, June 18).
(Because this is the only way for Albert to be certain that Bernard does not know)
The second statement tells us that the day number is one shared only by the two months eliminated in the first statement. So the only choices left are: July 16 (elimination of May 16), August 15 (May 15), August 17 (June 17).
(Because this is only way for Bernard to be able to figure the exact date only now)
The third statement tells us that it's a month that has only one day number in the set [16, 15, 17]. So it must be July 16.
(Because this is only way for Albert to be able to figure out the exact date only now)
 


soistes

Well-known member
Sep 12, 2012
2,643
Brighton
The first statement excludes that the birthday is in a month with a unique day number (May 19, June 18).
(Because this is the only way for Albert to be certain that Bernard does not know)
The second statement tells us that the day number is one shared only by the two months eliminated in the first statement. So the only choices left are: July 16 (elimination of May 16), August 15 (May 15), August 17 (June 17).
(Because this is only way for Bernard to be able to figure the exact date only now)
The third statement tells us that it's a month that has only one day number in the set [16, 15, 17]. So it must be July 16.
(Because this is only way for Albert to be able to figure out the exact date only now)

Is the correct answer (and reasoning)
http://www.theguardian.com/science/...rt-bernard-and-cheryls-birthday-maths-problem
 


jimbob5

Banned
Sep 18, 2014
2,697
A knows B does not know because the Month A has does not have a unique number. Therefore A knows it's not May or June. A tells B he knows B does not know so B knows it's not May or June. Therefore B knows the answer is X. B tells A he knows the answer so A thinks, the only way B could know the answer would be if the number is 16. If B had 14, it could be July OR August. B said he did not know the answer until he found out the month was June or July. A knows it's July and B knows it's 16. B knows its 16 th July because May and June are already eliminated and B knows there is no option for 16 in August. A knows it's July so when B says he, B knows the answer, he eliminates 14 because 14 could be August, which leaves just 16 for July.
 





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