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Manager  Joined: 25 Jul 2010
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Is positive integer n 1 a multiple of 3?  [#permalink]

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Question Stats: 39% (02:01) correct 61% (02:16) wrong based on 236 sessions

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Is positive integer n – 1 a multiple of 3?

(1) n^3 – n is a multiple of 3

(2) n^3 + 2n^2 + n is a multiple of 3

For 1, for following values of n, $$n^3 - n$$ becomes multiple of 3 : 2,3,4,5,6,7,8....

However, by plugging these values in n-1, we have different results. thus, not sufficient

For 2, following values of n gets the equation as a multiple of 3 :
2,3,5,6,7,8....
Its again not sufficient.

Combining 1 & 2, we again can't be conclusive. Thus,answer should be E.

Please write where am I wrong?
Math Expert V
Joined: 02 Sep 2009
Posts: 60494
Re: Positive integer n-1 multiple of 3?  [#permalink]

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Orange08 wrote:
Is positive integer n – 1 a multiple of 3?

(1) n^3 – n is a multiple of 3

(2) $$n^3 + 2n^2+ n$$ is a multiple of 3

For 1, for following values of n, $$n^3 - n$$ becomes multiple of 3 : 2,3,4,5,6,7,8....

However, by plugging these values in n-1, we have different results. thus, not sufficient

For 2, following values of n gets the equation as a multiple of 3 :
2,3,5,6,7,8....
Its again not sufficient.

Combining 1 & 2, we again can't be conclusive. Thus,answer should be E.

Please write where am I wrong?

Is positive integer n – 1 a multiple of 3?

(1) n^3 – n is a multiple of 3 --> $$n^3-n=n(n^2-1)=(n-1)n(n+1)=3q$$. Now, $$n-1$$, $$n$$, and $$n+1$$ are 3 consecutive integers and one of them must be multiple of 3, so no wonder that their product is a multiple of 3. However we don't know which one is a multiple of 3. Not sufficient.

(2) $$n^3 + 2n^2+ n$$ is a multiple of 3 --> $$n^3 + 2n^2+ n=n(n^2+2n+1)=n(n+1)^2=3p$$ --> so either $$n$$ or $$n+1$$ is a multiple of 3, as out of 3 consecutive integers $$n-1$$, $$n$$, and $$n+1$$ only one is a multiple of 3 then knowing that it's either $$n$$ or $$n+1$$ tells us that $$n-1$$ IS NOT multiple of 3. Sufficient.

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Re: Positive integer n-1 multiple of 3?  [#permalink]

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Orange08 wrote:
Is positive integer n – 1 a multiple of 3?

(1) $$n^3 – n$$ is a multiple of 3

(2) $$n^3 + 2n^2+ n$$ is a multiple of 3

For 1, for following values of n, $$n^3 - n$$ becomes multiple of 3 : 2,3,4,5,6,7,8....

However, by plugging these values in n-1, we have different results. thus, not sufficient

For 2, following values of n gets the equation as a multiple of 3 :
2,3,5,6,7,8....
Its again not sufficient.

Combining 1 & 2, we again can't be conclusive. Thus,answer should be E.

Please write where am I wrong?

(1) $$n^3-n = n(n^2-1) = n(n+1)(n-1)$$
This number will always be a multiple of three ... so not sufficient to answer our question

(2) $$n^3 + 2n^2+ n = n(n+1)^2$$
If this is a multiple of 3, then either n is a multiple of 3 or (n+1) is a multiple of 3. In either case, n-1 cannot be a multiple of 3.
Hence this is sufficient

In your solution above, notice that for all the n's in case (2) n-1 is not a multiple of 3.
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##### General Discussion
Senior Manager  S
Joined: 08 Dec 2015
Posts: 282
GMAT 1: 600 Q44 V27
Re: Is positive integer n 1 a multiple of 3?  [#permalink]

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But n-1 isn't mentioned in the 2) so how can we say something about it? how come that if n or n+1 is div by 3 then automatically n-1 is not?..
Math Expert V
Joined: 02 Sep 2009
Posts: 60494
Re: Is positive integer n 1 a multiple of 3?  [#permalink]

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iliavko wrote:
But n-1 isn't mentioned in the 2) so how can we say something about it? how come that if n or n+1 is div by 3 then automatically n-1 is not?..

Out of any 3 consecutive integers only 1 is a multiple of 3. We know it's either n or n + 1, therefore it's not n-1.
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Senior Manager  S
Joined: 08 Dec 2015
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GMAT 1: 600 Q44 V27
Re: Is positive integer n 1 a multiple of 3?  [#permalink]

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ahhhhh okok! now got it.. omg this is a tricky one....

Thank you Bunuel!
Intern  B
Joined: 30 Jul 2017
Posts: 19
Re: Is positive integer n 1 a multiple of 3?  [#permalink]

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Hello,

What is the approximate level of this question on 800 scale?

Regards.
Math Expert V
Joined: 02 Sep 2009
Posts: 60494
Re: Is positive integer n 1 a multiple of 3?  [#permalink]

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krikre wrote:
Hello,

What is the approximate level of this question on 800 scale?

Regards.

Check the original post for tags and other stats. It's a 700-level question.
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GMAT Club Legend  V
Joined: 12 Sep 2015
Posts: 4214
Re: Is positive integer n 1 a multiple of 3?  [#permalink]

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Top Contributor
1
Orange08 wrote:
Is positive integer n – 1 a multiple of 3?

(1) n³ – n is a multiple of 3

(2) n³ + 2n² + n is a multiple of 3

Target question: Is positive integer n – 1 a multiple of 3?

Statement 1: n³ – n is a multiple of 3
Let's do some FACTORING
n³ – n = n(n² - 1) = n(n + 1)(n - 1)
So, statement 1 tells us that n(n + 1)(n - 1) is a multiple of 3
Notice that (n-1), n and (n+1) are 3 CONSECUTIVE integers, and we know that 1 out of every 3 consecutive integers is a multiple of 3 (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.
So, ONE of the following n, (n + 1), or (n - 1) a multiple of 3, but WHICH ONE??
It COULD be the case that (n - 1) IS a multiple of 3, OR it could be the case that (n - 1) is NOT a multiple of 3
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: n³ + 2n² + n is a multiple of 3
More FACTORING
n³ + 2n² + n = n(n² + 2n + 1)
= n(n + 1)(n + 1)
So, statement 2 tells us that n(n + 1)(n + 1) is a multiple of 3
This means that EITHER n is a multiple of 3 OR (n + 1) is a multiple of 3
Let's examine both cases:

Case a: n is a multiple of 3
If n is a multiple of 3, then n - 1 CANNOT be a multiple of 3.
Why not?
Well, we already know that 1 out of every 3 consecutive integers is a multiple of 3
So, if n is a multiple of 3, the n+3 is also a multiple of 3 AND n+6 is a multiple of 3, AND n+9 is a multiple of 3, etc
Likewise, n-3 is a multiple of 3 AND n-6 is a multiple of 3, AND n-9 is a multiple of 3, etc.
Notice that n-1 is NOT among the possible multiples of 3
So, in this case, the answer to the target question is NO, n - 1 is NOT a multiple of 3

Case b: (n + 1) is a multiple of 3
If n+1 is a multiple of 3, then n - 1 CANNOT be a multiple of 3.
We'll use the same logic we used in case a above
1 out of every 3 consecutive integers is a multiple of 3
So, if n+1 is a multiple of 3, the n+1+3 (aka n+4) is also a multiple of 3 AND n+1+6 (aka n+7) is a multiple of 3, AND n+1+9 (aka n+10)is a multiple of 3, etc
Likewise, n+1-3 (aka n-2) is a multiple of 3 AND n+1-6 (aka n-5) is a multiple of 3, AND n+1-6 (aka n-5) is a multiple of 3, etc.
Notice that n-1 is NOT among the possible multiples of 3
So, in this case, the answer to the target question is NO, n - 1 is NOT a multiple of 3

IMPORTANT: for statement 2, there are only two possible cases, and in each case, the answer to the target question is the SAME: NO, n - 1 is NOT a multiple of 3
So, it MUST be the case that n - 1 is NOT a multiple of 3
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Cheers,
Brent
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