Home > math puzzle > (Another) Birthday problem puzzle

(Another) Birthday problem puzzle

I’d like to discuss one of many birthday problem math puzzles. The complete puzzle and the solution can be found here. But let me write the puzzle as follows.

You and your colleagues know that your boss A’s birthday is one of the following 10 dates:

Mar 4, Mar 5, Mar 8
Jun 4, Jun 7
Sep 1, Sep 5
Dec 1, Dec 2, Dec 8
A told you only the month of his birthday, and told your colleague C only the day. After that, you first said: “I don’t know A’s birthday; C doesn’t know it either.” After hearing what you said, C replied: “I didn’t know A’s birthday, but now I know it.” You smiled and said: “Now I know it, too.” After looking at the 10 dates and hearing your comments, your administrative assistant wrote down A’s birthday without asking any questions. So what did the assistant write?

When I first read this puzzle, I am confused. Let me just name the reader/myself  “B” in the puzzle above to avoid more confusion. This puzzle is written in some way that the reader is part of the story. Hence the reader, B, was told the month of A’s birthday but we as the reader do not know the month of A’s birthday. Perhaps it is just a part of puzzle that we have to solve, therefore it might not be a big deal.

However, the big deal is the fact that the solution that I found is different from the solution given. I thought I might get wrong, therefore I look for the solution. What happened was I could not understand the solution. I thought it is just me but it turns out there is someone else who has the same experience.

I would like to modify a little bit of the puzzle in order to make this puzzle a bit clearer, well at least for me.

A’s birthday is one of the following 10 dates:

Mar 4, Mar 5, Mar 8
Jun 4, Jun 7
Sep 1, Sep 5
Dec 1, Dec 2, Dec 8
We then have the following conversations between A’s friend.
C said: “I don’t know A’s birthday, but I know only the day (the date) of A’s birthday,”
B said: “Well, I know only the month A’s birthday.”
C said: “Now I know it.”
B said: “Now I know it, too.”

From the information given above, can you tell what is A’s birthday?

Perhaps, this version is better than the previous one. What do you think?

Categories: math puzzle Tags: ,
  1. May 13, 2011 at 5:23 pm

    kenapa bukan Juni dan Desember?

    • ivanky
      May 13, 2011 at 11:51 pm

      Puzzle aslinya jawabannya Sep 1 Ko, nah ini gw ga ngerti banget. Soalnya gua dapatnya Mar 4.
      Solusinya itu Juni sama Desember di ruled out, nah ini gw ga ngerti kenapa desember di ruled out, kalo juni di ruled out g ngerti sie

  2. May 16, 2011 at 6:05 am

    …gue googling dan gue ngerti kenapa Juni dan Desember di rule out…

    Kalimat pertama..”Gue gak tahu ulang tahun si A, dan gue yakin C enggak tau juga.” Ini artinya bulannya pasti Maret dan September, soalnya kalau Juni atau Desember harusnya ngomongnya, “Gue gak tahu ulang tahun si A, tapi mungkin si C tau kalau si C dapet tanggalnya 2 dan 7”


    • ivanky
      May 16, 2011 at 6:10 am

      Iya ko, uda ngerti kok, makanya wording nya gw ga nangkep, harusnya si B ngmgnya gini:

      I don’t know A’s birthday but I know 100% that C doesn’t know it either

      Soalnya yang gw pikir tuh, si C ngmg ke si A bahwa dia ga tahu

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