Prime number division

This topic has 1 expert reply and 6 member replies
ousek Junior | Next Rank: 30 Posts
Joined
07 Jul 2008
Posted:
20 messages
Target GMAT Score:
700+

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

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    Pranay Senior | Next Rank: 100 Posts Default Avatar
    Joined
    05 May 2009
    Posted:
    59 messages
    Thanked:
    1 times
    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
    Joined
    07 Jul 2008
    Posted:
    20 messages
    Target GMAT Score:
    700+
    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

    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.

    _________________
    If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com

    ousek Junior | Next Rank: 30 Posts
    Joined
    07 Jul 2008
    Posted:
    20 messages
    Target GMAT Score:
    700+
    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
    Joined
    21 Nov 2008
    Posted:
    21 messages
    Thanked:
    1 times
    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
    Joined
    04 Feb 2009
    Posted:
    319 messages
    Followed by:
    9 members
    Thanked:
    84 times
    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
    Joined
    10 Jul 2010
    Posted:
    2 messages
    Thanked:
    1 times
    Post Fri Mar 11, 2011 10:07 pm
    Thank you Suresh thats a very good explaination

    Best Conversation Starters

    1 AbeNeedsAnswers 30 topics
    2 amontobin 16 topics
    3 richachampion 12 topics
    4 NandishSS 8 topics
    5 rsarashi 5 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description Matt@VeritasPrep

    Veritas Prep

    74 posts
    2 image description GMATGuruNY

    The Princeton Review Teacher

    72 posts
    3 image description Rich.C@EMPOWERgma...

    EMPOWERgmat

    66 posts
    4 image description DavidG@VeritasPrep

    Veritas Prep

    65 posts
    5 image description Jay@ManhattanReview

    Manhattan Review

    55 posts
    See More Top Beat The GMAT Experts