Is positive integer n 1 a multiple of 3? (1) n^3 n is a

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

eschn3am · Answer

I'm getting E for this one.

1. n could be 2 or 4

2^3 - 2 = 6 
4^3 -4 = 60

2-1 is not a multiple of 3, but 4-1 is. INSUFFICIENT

2. again, both 2 and 4 work for this one. INSUFFICIENT

together we still don't know because 2 and 4 work for both.

Answer E

rgajare14 · Answer

From 1, n^3 - n = n(n^2 - 1) = n(n-1)(n+1) 
So n , n+1 or n -1 can be multiple of 3. But we dont know which..

From 2, n(n^2+2n+1) = n (n+1)^2 
So n or (n+1)^2 is a multiple of 3. Not suff.

From 1 and 2, not suff
Answer E

GMATBLACKBELT · Answer

S1: n(n^2-1)/3 --> n(n-1)(n+1)/3 plug in 5. yes this is a multiple of 3 but n-1 is not. make n=7. then n-1 is a multiple of 3 Insuff

2: n(n^2+2n+1) --> n(n+1)(n+1)/3 --> This essnetially says that n must be divisible by 3 or n+1 must be divisble by 3. However we know that n-1 cannot be divisible by 3. Try any value in here and ul see why.

I say B.

ashkrs · Answer

B

A - n(n-1)(n+1) divisible by 3. Any of them can be divisible .So InSUFF

B - n(n+1)(n+1) divisible by 3 . Any of them n or n+1 can be divisible by 3. That certainly means n-1 will be not be divisible.

buffdaddy · Answer

1. When presented with three consecutive integers (n-1),(n),(n+1) one of the thre will be divisible by 3, but we do not know if n-1 is the multiple of 3 in this case. insuff

2. n or n+1 is divisible by 3. not sure. But in either case since n2 is within +-2 integers of n and n+1, it definitely isn't divisible by 3.
suff

B

dominion · Answer

B, you only got the question wrong because the answers are represented within the   function. The "answer" is 81, BUT, the answer choices are          ... and since   = 81. the answer is  .

jingy77 · Answer

The people who chose B seems to be correct. This one is tricky.

Is positive integer n 1 a multiple of 3? (1) n^3 n is a

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