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Is N odd?

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kamalakarthi Rising GMAT Star Default Avatar
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Is N odd? Post Mon Oct 06, 2014 8:42 am
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    Is the integer N odd ?

    1. n^2-2n is not a multiple of 4.
    2. N is a multiple of 3.

    In this above question, I am not able to understand how statement 1 is sufficient.
    Following is my analysis :-

    when N is 2, I get 2^2 - 2(2) which is 0 and it is not multiple of 4 but N is even.
    When N is 3, The value is get 9-6 =3 which is also not multiple of 4 but N is odd.

    With the above analysis, I thought statement 1 is NOT sufficient.

    I can undertstand that statement 2 will not be sufficient because n can be 3 or 6.

    Can you please help me understand the statement 1.

    Thank you.
    -karthik.

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    fcabanski Really wants to Beat The GMAT!
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    Post Mon Oct 06, 2014 10:05 am
    A multiple of 4 is the result of multiplying 4 by an integer. 0 is an integer.

    4*0 = 0. 0 is a multiple of 4.

    0 is a multiple of every number. 0 is not a factor of any number.

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    Post Mon Oct 06, 2014 10:58 am
    kamalakarthi wrote:
    Is the integer n odd?

    1. n² - 2n is not a multiple of 4.
    2. n is a multiple of 3.
    Target question: Is the integer n odd?

    Given: n is an INTEGER

    Statement 1: n² - 2n is not a multiple of 4.
    Factor to get: n(n - 2) is NOT a multiple of 4

    Underlying concepts:
    Integer n is 2 greater than n-2
    If n is ODD, then n-2 is also ODD, so n(n - 2) = (ODD)(ODD) = ODD. In this case, n(n-2) cannot be divisible by 4
    If n is EVEN, then n-2 is also EVEN, so n(n - 2) = (EVEN)(EVEN) = EVEN. More importantly, n and n-2 are CONSECUTIVE even integers, and the product of two CONSECUTIVE even integers is always a multiple of 4


    Statement 1 tells us that n(n - 2) is NOT a multiple of 4
    So, it cannot be the case that n is EVEN
    In other words, it MUST be the case that n is ODD
    Since we can answer the target question with certainty, statement 1 is SUFFICIENT

    Statement 2: n is a multiple of 3
    There are several values of n that satisfy this condition. Here are two:
    Case a: n = 3, in which case n n is ODD
    Case b: n = 6, in which case n n is EVEN
    Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

    Answer = A

    Cheers,
    Brent

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    kamalakarthi Rising GMAT Star Default Avatar
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    Post Mon Oct 06, 2014 11:08 am
    Brent , Thank you for your reply.

    I can understand that if N is even then N-2 is also even and will be multiple of 4. I think I am missing a point when it comes to 0.

    If I choose N as 2, then when I do N-2, the result is 0 which is not a multiple of 4.

    I have another doubt in your explanation. "the product of two CONSECUTIVE even integers is always a multiple of 4 "

    My understanding is 0 as an even integer and if I take 0 and 2, it is not multiple of 4. Am I right?

    I know I am missing a point but trying to understand.

    Post Mon Oct 06, 2014 11:24 am
    Hi kamalakarthi,

    There are some Number Property rules about the number 0 that every Test Taker should know:

    1) 0 is EVEN
    2) 0 is a "NULL" value; it is neither positive nor negative
    3) 0 IS a multiple of EVERY integer.

    So, when you multiply 0 and 2, the product = 0, which IS even.

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    fcabanski Really wants to Beat The GMAT!
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    Post Mon Oct 06, 2014 11:31 am
    "If I choose N as 2, then when I do N-2, the result is 0 which is not a multiple of 4. "

    As I mentioned in the first reply, 0 is a multiple of 4.

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    Post Sun Oct 12, 2014 10:12 pm
    fcabanski wrote:
    "If I choose N as 2, then when I do N-2, the result is 0 which is not a multiple of 4. "

    As I mentioned in the first reply, 0 is a multiple of 4.
    One important property that isn't intuitive to most students is that 0 is a multiple of EVERY integer.

    Here's how we know.

    (integer x) * (any other integer) = (a multiple of integer x)

    Since 0 can be "any other integer", we have

    x * 0 = a multiple of x

    or

    0 = a multiple of x

    Hence 0 is a multiple of EVERY integer. You will encounter this on an official GMAT problem sometime, be it the OG, a problem from mba.com, or your actual exam, so it's a good thing to remember.

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