Welcome to Beat The GMAT, the world's largest GMAT & MBA forum. Please join our rich community of friendly members. Get free help from
experts who teach at the leading GMAT & MBA prep companies — Kaplan, Veritas Prep, Manhattan GMAT and Stacy Blackman Consulting.

Announcement: Apply for the Fall 2008 Beat The GMAT Scholarship, featuring $20,000 in prizes.

DS prep


 
Post new topic   Reply to topic    Beat The GMAT Forum Index -> GMAT Math -> Data Sufficiency
View previous topic :: View next topic  
Author Message
bww
Rising GMAT Star


Joined: 29 Apr 2007
Posts: 55

Thanks given: 0
Thanked 0 times in 0 posts

Location: MA
PostPosted: Sun May 13, 2007 3:28 pm    Post subject: DS prep Reply with quote
14. If x and y are integers, is xy + 1 divisible by 3?

(1) When x is divided by 3, the remainder is 1.
(2) When y is divided by 9, the remainder is 8.

Thx
Back to top


View user's profile Send private message
jayhawk2001
Moderator


Joined: 28 Jan 2007
Posts: 789

Thanks given: 0
Thanked 11 times in 11 posts

Location: Silicon valley, California
PostPosted: Sun May 13, 2007 3:36 pm    Post subject: Re: DS prep Reply with quote
bww wrote:
14. If x and y are integers, is xy + 1 divisible by 3?

(1) When x is divided by 3, the remainder is 1.
(2) When y is divided by 9, the remainder is 8.

Thx


Is it C ?

1 - insufficient. x = 3t+1.
[ (3t+1)*y + 1 ] / 3
= (3ty + y + 1) / 3

So, divisibility depends on the value of y

2 - insufficient. y = 9s + 8
(xy + 1) / 3
= (9sx + 8x + 1) / 3

Again, depends on value of x

Together,

(3t+1)*(9s+8 ) + 1
= 27ts + 9s + 24t + 8 + 1
= 27ts + 9s + 24t + 9

All the terms above are divisible by 3 and hence is sufficient.
Back to top


View user's profile Send private message
bww
Rising GMAT Star


Joined: 29 Apr 2007
Posts: 55

Thanks given: 0
Thanked 0 times in 0 posts

Location: MA
PostPosted: Sun May 13, 2007 4:43 pm    Post subject: Reply with quote
yes, OA is C

thx
Back to top


View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    Beat The GMAT Forum Index -> GMAT Math -> Data Sufficiency All times are GMT - 7 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum



"GMAT" and other GMAC™ trademarks are registered trademarks of the Graduate Management Admission Council™. The Graduate Management Admission Council™ does not endorse, nor is it affiliated in any way with the owner or any content on this website. The opinions expressed here are solely those of the author or those of the members of this website. Copyright © 2008 BTG Test Prep, LLC. Powered by phpBB © 2001, 2005 phpBB Group.