MGMAT

[This topic has 2 member replies]

Free $100 Amazon.com Gift Card - Buy a GMAT course using a Beat The GMAT discount code between Mar 8-22 and get a $100 Amazon.com Gift Card. Learn more!

« View previous topic

View next topic »

f2001290
GMAT Destroyer!

Joined: 10 Mar 2007
Posts: 400

Thanks given: 0
Thanked 0 times in 0 posts

Topic: MGMAT

Mon May 28, 2007 10:40 am

	Elapsed Time:	00:00		Why a timer is critical to improving your score
	Lap #[LAPCOUNT] ([LAPTIME])

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 10:11 pm; edited 1 time in total

jayhawk2001
Moderator

Joined: 28 Jan 2007
Posts: 789

Thanks given: 0
Thanked 15 times in 14 posts
Location: Silicon valley, California

Mon May 28, 2007 7:06 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 = 3m n or n+1 is a multiple of 3. So, n-1 cannot be a multiple of 3. Sufficient. My vote for B.