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Cheryl's birthday

  • 14-04-2015 2:47pm
    #1
    vanderlyle
    Registered Users, Registered Users 2 Posts: 339 ✭✭


    _82299333_24e94c5d-dc7b-4590-8f46-29d0da74c8b8.jpg

    Popular problem doing the rounds on media today... took me a while!


Comments

  • #2
    jimmii
    Registered Users, Registered Users 2 Posts: 14,810 ✭✭✭✭jimmii


    I loved this its very clever. I think there is zero chance I would have got this when I was 14! I would have read it thought wtf is that about and moved on to the next question!


  • #3
    retro:electro
    Registered Users, Registered Users 2 Posts: 21,039 ✭✭✭✭retro:electro


    I think you would need to be telepathic to know the answer to that one. I still don't get it even when I saw the formula


  • #4
    vanderlyle
    Registered Users, Registered Users 2 Posts: 339 ✭✭vanderlyle


    jimmii wrote: »
    I loved this its very clever. I think there is zero chance I would have got this when I was 14! I would have read it thought wtf is that about and moved on to the next question!

    +1 to all of that


  • #5
    jimmii
    Registered Users, Registered Users 2 Posts: 14,810 ✭✭✭✭jimmii


    anna080 wrote: »
    I think you would need to be telepathic to know the answer to that one. I still don't get it even when I saw the formula

    Yeh takes a while to get your heard round it!


  • #6
    meep
    Registered Users, Registered Users 2 Posts: 1,291 ✭✭✭meep


    anna080 wrote: »
    I think you would need to be telepathic to know the answer to that one. I still don't get it even when I saw the formula

    Where did you see the formula?

    Here's the logic I used to work it out...
    Albert has the month
    Bernard has the day

    It can't be May 19th or June 18th as they are unique days and Albert would have known it immediately.

    When Albert says he doesn't know, Bernard can see it's not either of those dates and by virtue of the fact that he then immediately knows, it must be June 17th.

    (That's the only month for which there's only one remaining possibility)


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  • #7
    jimmii
    Registered Users, Registered Users 2 Posts: 14,810 ✭✭✭✭jimmii


    meep wrote: »
    Where did you see the formula?

    Here's the logic I used to work it out...
    Albert has the month
    Bernard has the day

    It can't be May 19th or June 18th as they are unique days and Albert would have known it immediately.

    When Albert says he doesn't know, Bernard can see it's not either of those dates and by virtue of the fact that he then immediately knows, it must be June 17th.

    (That's the only month for which there's only one remaining possibility)

    Its not
    June 17th
    its
    July 16th
    Alberts first line: this rules out May & June because he has said he definitely knows Bernard couldn't know purely from the number which he couldn't say if it was May or June as then Bernard might know the date purely from the number (if he happened to have 18 or 19)

    Bernards line: From that Bernard can deduce that May & June are out he holds 16 so now he obviously knows that the date is July 16th

    Alberts second line: Because Bernard has said he now knows what it is that means it can't be 14 (as then he wouldn't know whether it was July or August) and it couldn't be August 15th or 17th because Albert was already told it was July so the only date it could be is July 16th


  • #8
    Nidgeweasel
    Registered Users, Registered Users 2 Posts: 2,621 ✭✭✭Nidgeweasel


    meep wrote: »
    Where did you see the formula?

    Here's the logic I used to work it out...
    Albert has the month
    Bernard has the day

    It can't be May 19th or June 18th as they are unique days and Albert would have known it immediately.

    When Albert says he doesn't know, Bernard can see it's not either of those dates and by virtue of the fact that he then immediately knows, it must be June 17th.

    (That's the only month for which there's only one remaining possibility)



    That's wrong though.


  • #9
    meep
    Registered Users, Registered Users 2 Posts: 1,291 ✭✭✭meep


    Nidgeweasel wrote: »
    That's wrong though.

    Must try harder!

    (though that answer still makes complete sense to me :-( )


  • #10
    jimmii
    Registered Users, Registered Users 2 Posts: 14,810 ✭✭✭✭jimmii


    meep wrote: »
    Must try harder!

    (though that answer still makes complete sense to me :-( )

    The important bit is to keep in mind what they were told. If that Albert had been give June then he wouldn't have been able to say that first line so that means all the dates in June/May are out not just the 18th & 19th.


  • #11
    vanderlyle
    Registered Users, Registered Users 2 Posts: 339 ✭✭vanderlyle


    ALBERT wrote:
    I know that Bernard does not know

    means
    I know that Bernard has not been given date 18th or date 19th, because I have been given (one of) July or August


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  • #12
    ZorbaTehZ
    Registered Users, Registered Users 2 Posts: 2,149 ✭✭✭ZorbaTehZ


    Here is a more interesting variant.

    Let x,y be two integers greater than 1 such that x+y is less than 100. Charlie knows the value of x+y, and Bob knows the value of xy.

    Bob: I don't know what x and y are.
    Charlie: I knew that you didn't.
    Bob: Now I know the value of x and y.
    Charlie: Now I do as well.


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