MGMAT

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MGMAT

by f2001290 » Mon May 28, 2007 9:40 am
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

I will go with B. Any explanations???
Last edited by f2001290 on Mon May 28, 2007 9:11 pm, edited 1 time in total.

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Re: MGMAT

by jayhawk2001 » Mon May 28, 2007 6:06 pm
f2001290 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

I will go with D. Any explanations???
How did you get D ?

1 - n(n^2-1) = 3k
n*(n+1)*(n-1) = 3k
So, n, n+1 or a n-1 is a multiple of 3. Insufficient.

As an example take n = 3 and n = 4. n^3-n is a multiple of 3 for
both values of n but only n=4 will yield n-1 as a multiple of 3.

2 - n*(n+1)^2 = 3*m
n or n+1 is a multiple of 3. So, n-1 cannot be a multiple of 3.
Sufficient.

My vote for B.

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by f2001290 » Mon May 28, 2007 8:47 pm
Thanks Jay- Typing mistake. B is the right option