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  1. #1

    The birthday puzzle (DENISE'S REVENGE!)

    Having been defeated by round 1 I saw a chance to reclaim my honor with The Birthday Puzzle (DENISE'S REVENGE). I haven't read the answer on this one and will post again if I work it out. Don't forget to use spoiler tags if you get it.

    Albert, Bernard and Cheryl became friends with Denise, and they wanted to know when her birthday is. Denise gave them a list of 20 possible dates.

    17 Feb 2001, 16 Mar 2002, 13 Jan 2003, 19 Jan 2004
    13 Mar 2001, 15 Apr 2002, 16 Feb 2003, 18 Feb 2004
    13 Apr 2001, 14 May 2002, 14 Mar 2003, 19 May 2004
    15 May 2001, 12 Jun 2002, 11 Apr 2003, 14 Jul 2004
    17 Jun 2001, 16 Aug 2002, 16 Jul 2003, 18 Aug 2004
    Denise then told Albert, Bernard and Cheryl separately the month, the day and the year of her birthday respectively.
    The following conversation ensues:

    • Albert: I don’t know when Denise’s birthday is, but I know that Bernard does not know.
    • Bernard: I still don’t know when Denise’s birthday is, but I know that Cheryl still does not know.
    • Cheryl: I still don’t know when Denise’s birthday is, but I know that Albert still does not know.
    • Albert: Now I know when Denise’s birthday is.
    • Bernard: Now I know too.
    • Cheryl: Me too.

    So, when is Denise’s birthday?

  2. #2
    I think I got it. I tried to go through it line by line and write down my reasoning as I deleted dates.

    Spoiler!

  3. #3
    Lazy Idle Couch Potato Heptagon_ru's Avatar
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    Thanks! Nice brain rust cleaner

  4. #4
    Worst cooperation evar. They could have told each other what they know and the problem is solved. ;p

  5. #5
    Lazy Idle Couch Potato Heptagon_ru's Avatar
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    Ah, but it is not said that they are friends (Albert, Bernard and Cheryl). Probably they hate each other and hoped to know the date only by themselves. And that Denise probably wanted to found out who is the smartest one. Alas, they all kinda failed.

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