Is positive integer n 1 a multiple of 3?

Question

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?

Bunuel · Accepted Answer

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.

Answer: B.

shrouded1 · Answer

(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

Answer should be (b)

In your solution above, notice that for all the n's in case (2) n-1 is not a multiple of 3.

iliavko · Answer

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?..

Bunuel · Answer

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.

iliavko · Answer

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

Thank you Bunuel!

krikre · Answer

Hello,

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

Regards.

Bunuel · Answer

Check the original post for tags and other stats. It's a 700-level question.

BrentGMATPrepNow · Answer

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

Answer: B

Cheers,
Brent

jim441 · Answer

Bunuel   chetan2u , can you please clarify st 2 for n-1=3 (n=4)?

Bunuel · Answer

From (2) we get that either n or n + 1, is a multiple of 3, thus it cannot be n - 1. Hence, your choice of n - 1 does not satisfy (2) and therefore is invalid.

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Is positive integer n 1 a multiple of 3?

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