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A question from Gmat prep

This topic has 4 member replies
banona Senior | Next Rank: 100 Posts Default Avatar
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A question from Gmat prep

Post Fri Mar 02, 2007 5:38 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    I had the following DS question in my Gmatprep test. It's just confusing me as I don't think the answer is C.
    The question says, x and y are integers, is x*(y+1) even ?

    1- x and y are prime numbers
    2 - x > 7

    my answer was A, but Gmatprep said C

    Did anybody encounter the same question ?

    Thank you

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    Neo2000 Legendary Member
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    Post Fri Mar 02, 2007 6:39 pm
    Prime numbers are
    2,3,5,7,11,

    From 2
    X at the very least has to be = 11
    If y = 2
    11*(2+1) = Odd
    FOr anything else
    It will beome Even

    jayhawk2001 Community Manager
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    Post Sat Mar 03, 2007 10:10 am
    I can't see how the answer can be C.

    1 - Insufficient. As an example,
    x = 3, y = 2 implies x(y+1) is odd
    x = 2, y = 3 implies x(y+1) is even

    2 - Insufficient. x can be even or odd

    Combining 1 and 2, we can infer that x is odd.

    If y is 2, x(y+1) is odd
    If y is 3,5,7 ... x(y+1) is even

    So, I'm inclined towards E.

    Vasudha Senior | Next Rank: 100 Posts Default Avatar
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    Post Sun Mar 11, 2007 1:45 pm
    I did get this question when I took the practice test

    Have u written "-x and y are prime numbers" and "-x>7"?
    As far as I remember, the statements were "x and y are prime" and "x>7"

    Moreover primes are always positive.

    Vasudha Senior | Next Rank: 100 Posts Default Avatar
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    Post Sun Mar 11, 2007 1:57 pm
    So if you consider the first statement
    "x and y are prime"
    x(y+1)

    for x=2 and y=3 , 2*4=Even
    for x=3 and y=2, 3*3=Odd
    Hence, insufficient

    Second Statement "x>7" is insufficient coz x can be even making the product even or x and y can both be odd making the product odd.

    Together Also
    x=11,13....
    y=2,3,5 etc

    Hence for x=11 y=2, 11*3 = odd
    for x=11 y=3, 11*4 =even

    Hence the answer should be E

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