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Maths problem
- Thread starter sten_super
- Start date
sten_super
Brain Surgeon
- Thread starter
- #2
Okay I've changed my mind. Now I think it's 16th July. Good tester, although it's much more about logic than maths.
Moshe Gariani
Well-known member
- Mar 10, 2005
- 12,202
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.
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.
Vegas Seagull
New member
- Jul 10, 2009
- 7,782
June 17
I agree
Moshe Gariani
Well-known member
- Mar 10, 2005
- 12,202
Can't be June 17
Bad Ash
Unregistered User
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.
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.
KZNSeagull
Well-known member
Yes 16th July. Got there eventually.
Exactly.
sten_super
Brain Surgeon
- Thread starter
- #10
It was ruling out May and June that I missed first time round - I just excluded 18 and 19 instead.
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.
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.
SweatyMexican
Well-known member
- Mar 31, 2013
- 4,155
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.
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
- 5,026
I think its the 16th July
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)
(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
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.
happypig
Staring at the rude boys
I got October 4th