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Prime number division

This topic has 1 expert reply and 6 member replies
ousek Junior | Next Rank: 30 Posts
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Prime number division

Post Mon May 11, 2009 11:36 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Hi,

    I got this question from the ETS paper-based of GMAT #37 :

    If n is a prime number greater than 3, what is the
    remainder when n is divided by 12?

    (A) 0
    (B) 1
    (C) 2
    (D) 3
    (E) 5

    Confused Does anyone have an idea of the answer ?


    Ousek

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    Pranay Senior | Next Rank: 100 Posts Default Avatar
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    Post Tue May 12, 2009 1:19 am
    ousek wrote:
    Hi,

    I got this question from the ETS paper-based of GMAT #37 :

    If n is a prime number greater than 3, what is the
    remainder when n is divided by 12?

    (A) 0
    (B) 1
    (C) 2
    (D) 3
    (E) 5

    Confused Does anyone have an idea of the answer ?


    Ousek
    I am not sure .. but have narrowed down to two options B and E.

    Not sure between the two. Please post the answer once someone reaches the answer.

    ousek Junior | Next Rank: 30 Posts
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    Post Tue May 12, 2009 3:27 am
    The official answer is B.
    According to me, it is not the only answer possible, as you discovered.
    R(17/12)=5
    R(13/12)=1

    Perhaps is the question wrongly designed. Further prospection on the subject gave me the following rules:
    Arrow If n is a prime number greater than 3, then the remainder of (n^2)/12 is 1.
    This is the only one explanation I see. I even so asked to see if I felt in the analysis. Actually, this test sheet is marked "proofed" by GMAC... Confused
    Curious...

    What is your opinion ?

    Another explanation ?
    Ousek

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    Ian Stewart GMAT Instructor
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    Post Tue May 12, 2009 4:15 am
    In the version of the paper test that I have, it does ask for the remainder when n^2 is divided by 12, and not when n is divided by 12. If the question asks about n, of course there is more than one right answer, which never happens on the GMAT.

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    ousek Junior | Next Rank: 30 Posts
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    Post Tue May 12, 2009 4:24 am
    Ian Stewart wrote:
    In the version of the paper test that I have, it does ask for the remainder when n^2 is divided by 12, and not when n is divided by 12.
    Not in mine... it does clearly ask for R(n/12), which makes no sense.

    Nevertheless, thank for your confirmation, Ian !


    Best regards,

    Umar82 Junior | Next Rank: 30 Posts Default Avatar
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    Post Tue May 12, 2009 2:29 pm
    The answer is (E)

    13/12 gives remainder of 1

    17/12 gives remainder 5

    29/12 gives remainder 5

    go with the one that occur the most in my opinion, however the wording of this questions seems wrong

    sureshbala Master | Next Rank: 500 Posts
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    Post Tue May 12, 2009 6:31 pm
    The question has to be this....

    If n is a prime number greater than 3, what is the remainder when n^2 is divided by 12.

    Any prime number greater than 3 can be expressed in the form of 6K+1 or 6k-1.

    So n^2 = 36k^2 + 12k + 1 or 36k^2 -12k +1

    So it is now clear that in either case the remainder when n^2 is divided by 12 is 1.


    Of course you can always consider examples and finish this as the options do not contain "Cannot be determined"

    Thanked by: shreeuec
    shreeuec Newbie | Next Rank: 10 Posts Default Avatar
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    Post Fri Mar 11, 2011 10:07 pm
    Thank you Suresh thats a very good explaination

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