Someone with a Masters degree in reasoning and logic please help me

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kendo

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#1283208 14-Apr-2015 20:27

NonprayingMantis:
Rickles: Nonpraying mantis ... OK, got that, and assumed that to logically be the case, BUT there are also non-unique dates over several months.

I must be missing something here

correct, but if the answer was in a month that contains a unique day number then, no matter which date it actually is, then Albert cannot be certain that Bernard doesn't know. (because he only knows the month, not that date)

e.g. if Albert was told "May" then he knows it could be either 15th 16th or 19th. Since he doesn't know which one it is, then he cannot state "Bernard doesn't know the answer" because there is a chance Bernard does know the answer.

so the only months where Albert can definitely state "Bernard does not know" are months that contain no unique day numbers.

Oh yea, makes sense. If he is going first and knows it cant be a unique date(since his month has none), then he will know the other cannot possibly know it. (it cant be a month with a unique date).

NonprayingMantis

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#1283213 14-Apr-2015 20:33

Rickles: >so the only months where Albert can definitely state "Bernard does not know" are months that contain no unique day numbers.<

Agreed, but I am at loss to see how any of the remaining months/dates may be also eliminated.

so at this point you are onto the second step.

We've eliminated the months containing unique dates.
That leaves just July and August. At this point it could be any of the days in these months. we now have a smaller set of dates to play with.

Next statement: Bernard: "I didn't know before...."

Great, that confirms it isn't one of the unique dates

"...but now I do"

for that to be true, it must a date that does not appear in both July and August (if it appeared in both, Bernard wouldn't know the answer) That rules out the 14th

That leaves just three options: July 16th, August 15th, August 17th.
With these three options Bernard knows the answer. He already knew the day of the month, and since each day only appears once in this list, he can deduce the final answer.

final statement:

Albert: " Now I know the answer."

We know Albert knew the month from the start, but he now knows the day is one of the three options above. If the answer was either of the August dates, Albert wouldn't know the final answer (from his perspective, it could be August 15th or 17th).
The only way for him to state that he knows the answer is if it lies in a Month with only 1 option,which is July. so he now knows it's July 16th

nathan

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#1283216 14-Apr-2015 20:34

Geektastic:
joker97:

I don't get it.

1) 2 separate persons got told each the month, the other the day of a birthday.

2) A states he doesn't know the birthday but he is sure B doesn't know.

3) B says he knows the birthday.

4) A deduces if B knows then A must know it too.

WTF

Ignoring the actual question, the English is a disgrace.

Its Singlish

Rickles

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#1283285 14-Apr-2015 21:35

NonprayingMantis ... your quiet, considered and full responses are really appreciated. I still do not get "it" at all, but will try again tomorrow

tangerz

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#1283365 15-Apr-2015 00:14

Here's the logic:

Before any statements:

Albert knows month only.
Bernard knows day number only.
We know neither month nor day number.

Albert: I don't know when Cheryl's birthday is, but I know Bernard does not know too.

As others have pointed out, by Albert stating 'I know Bernard does not know too' he eliminates May and June as the possible month as they both contain a unique 'day number' (18th and 19th) which would mean Bernard *might* know the birthday if in those months.

But in eliminating May and June it also rules out the duplicate 15th, 16th and 17th. *This allows Bernard to determine the correct month*

After first statement:

Albert knows month only.
Bernard knows month and day number.
We know month is either July or August.

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

The statement 'At first I don't know' confirms the day number was a duplicate (14th, 15th, 16th, 17th) and not unique (18th, 19th).

The statement ' but I know now' eliminates the 14th as the day number as Bernard still wouldn't know if it was the 14th (as both July 14th and August 14th are possibilities).

But eliminating the 14th leaves only one date in July (16th) and two in August (15th and 17th). *This allows Albert to determine the correct day number*

After second statement:

Albert knows month and day number.
Bernard knows month and day number.
We know date is either July 16th, August 15th or August 17th.

Albert: Then I also know when Cheryl's birthday is.

The statement 'Then I also know' eliminates August as the month as Albert couldn't know which of the two August dates is correct. He can only know if the month he was told is July.

After the third statement:

Albert knows month and day number.
Bernard knows month and day number.

WE KNOW CHERYL'S BIRTHDAY IS JULY 16TH!

BTR

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#1284420 15-Apr-2015 08:35

All I read from that is that are in the "Friend Zone" and it sounds like she has skeletons in her closet as she won't reveal her birthday which means she's possibly lying about her age.

Athlonite

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#1285199 15-Apr-2015 22:35

Samsung

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JWR

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#1285241 15-Apr-2015 23:57

I like these type of questions.

I can understand why Singapore puts them in their exam questions.

It is a lot how Science works.

Write down the basics of the problem.

Then eliminate what can't be true.

nakedmolerat

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#1285244 16-Apr-2015 00:08

It is an olympiad question - done by many students in the south east Asia region.

It's not just Singapore.

JWR

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#1285246 16-Apr-2015 00:16

nakedmolerat: It is an olympiad question - done by many students in the south east Asia region.

It's not just Singapore.

It isn't really important where the problem comes from.

It is a universal problem.

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