Brain exercise

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7 years 2 months ago #39487 by Organix
Brain exercise was created by Organix
This is (a more clearly written version of) a logic question from an exam put to 14/15 year old Singaporean students that has gone viral online over the past few days. See if you can deduce the date of Cheryl's birthday, without resorting to Google [;)]
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.

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

Albert: Well, now I know, too!

When is Cheryl’s birthday?

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7 years 2 months ago #505314 by Organix
Replied by Organix on topic Brain exercise
Apologies for the formatting hieroglyphics. It looks like I might have 'broken' the forum as it won't allow me to correct/delete these [:0]

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7 years 2 months ago #505316 by Anakei
Replied by Anakei on topic Brain exercise
I saw this in the newspaper the other day. No wonder they are running rings around us in education if that is the level of thinking they aspire to!! I had to look at the answer as I had NO idea how to work it out, but once you see it you can follow the logic.
Well sort of......:confused:

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7 years 2 months ago #505317 by kai
Replied by kai on topic Brain exercise
July 16th

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7 years 2 months ago #505318 by WillEyre
Replied by WillEyre on topic Brain exercise

kai;511678 wrote: July 16th

Big deal. But, without 'proof', how do we know you're right?
But please explain for us lesser mortals (in your own words, not plagiarised) just how you came by this answer - otherwise it's no different from finding the 'solution' in the back of the maths book.

I liked Occam's Razor so much, I bought the company.

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7 years 2 months ago #505319 by WillEyre
Replied by WillEyre on topic Brain exercise
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 May 16 May 19
June 17 June 18
July 14 July 16
August 14 August 15 August 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 don't know when Cheryl's birthday is, but I know now.
Albert: Then I also know when Cheryl's birthday is.
So when is Cheryl's birthday?

The above is how I saw it when it first appeared, the other day.

It is actually a lot more confusing and difficult in this form than it is in the posting by Organix, because in the above we only know that Albert and Bernard are told 'separately,the month and day of her birthday, respectively'
You've then got to decipher that gibberish before you can even begin. In other words you are committed to work out the question (which is somewhat ambiguous) before you can attempt the answer.

It's actually pretty simple when you know that what is actually meant is that;
Cheryl told Albert the month,but only the month of her birthday.
Then she told Bernard the day, but only the day.

I liked Occam's Razor so much, I bought the company.

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7 years 2 months ago #505321 by kai
Replied by kai on topic Brain exercise
If you want a long worded explanation, here it is.
www.theguardian.com/science/alexs-advent...rthday-maths-problem

My short explanation is I used to have a hobby buying and solving logic problem books.

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7 years 2 months ago #505323 by WillEyre
Replied by WillEyre on topic Brain exercise

kai;511682 wrote: If you want a long worded explanation, here it is.
www.theguardian.com/science/alexs-advent...rthday-maths-problem

My short explanation is I used to have a hobby buying and solving logic problem books.

No, I didn't want a long-winded explanation.
Seeing that you offered an answer, unsubstantiated, I was interested to see if you could explain it, briefly, in your own words, unplagiarised.

I liked Occam's Razor so much, I bought the company.

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7 years 2 months ago #505324 by kai
Replied by kai on topic Brain exercise

WillEyre;511684 wrote: No, I didn't want a long-winded explanation.
Seeing that you offered an answer, unsubstantiated, I was interested to see if you could explain it, briefly, in your own words, unplagiarised.

My own explanation, would be it can't be that, it can't be that etc, so therefore the only one left is that.
If you do logic problems often, they just become obvious.

Why do you have an issue with having to substantiate a logic problem. The answer is either correct or it is not. There is no argument needed eg 2+3=5 why? cos it does.

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7 years 2 months ago #505359 by Baroque
Replied by Baroque on topic Brain exercise
The answer is purple, because aliens don't wear hats. :p

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7 years 2 months ago #505360 by WillEyre
Replied by WillEyre on topic Brain exercise

Baroque;511724 wrote: The answer is purple, because aliens don't wear hats. :p

Prove it!

I liked Occam's Razor so much, I bought the company.

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7 years 2 months ago #505361 by kindajojo
Replied by kindajojo on topic Brain exercise
In the first instance Albert cannot know the birthday, since he only knows the month and there are at least two dates each month
Bernard tells him he does not know......since Bernard knows the date....this excludes two dates the 19th and the 18 th since these only appear once .

If Albert does still not know ...then we can exclude JUNE since this month now only has one day
.........and that's where it all falls apart....

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7 years 2 months ago #505395 by Organix
Replied by Organix on topic Brain exercise
Put as simply as I can, Albert knows the month and Bernard knows the day.

For Albert to say that Bernard (definitely) doesn't know rules out May and June because both have a day (19 & 18 respectively) that is a possibility that occurs only once on Cheryl's list. If Albert had been told May or June he couldn't have categorically ruled out Bernard knowing.

Using Albert's logic above Bernard's revelation indicates that the day he was told occurs in ONLY July OR August which therefore rules out 14 as it is a possibility for both (and he has no way of knowing which month if it was 14). The other days (15, 16 & 17) occur only once so he can 'match' the month by day alone.

For Albert to now (definitely) know means the month he was told must be July as if it was August he would still be left with not knowing whether it was 15 or 17, so rule out August.

Therefore the answer is July 16th.

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7 years 2 months ago #505408 by Baroque
Replied by Baroque on topic Brain exercise

WillEyre;511725 wrote: Prove it!

You prove it isn't! :p

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7 years 2 months ago #505409 by kindajojo
Replied by kindajojo on topic Brain exercise
Why does it rule out may....since there are three dates in May 15,16 and 19
We can rule out the 19 since it only occurs once...but why can we rule out the 15 and the 16 since these dates occur in both July and August.

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