Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Manhattan GMAT CAT Math


 Page 1 of 1 [ 6 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

MGMAT Problem

Author Message
dewanak
 Post subject: MGMAT Problem
 Post Posted: Sat Jul 03, 2010 6:26 pm 
Offline
Students


Posts: 16
If a – b > a + b, where a and b are integers, which of the following must be true?

I. a < 0
II. b < 0
III. ab < 0

(1) I only

(2) II only

(3) I and II only

(4) I and III only

(5) II and III only

as per the MGMAT answer is (2)

I am ok with b<0 which comes directly from the equation but i say (3) is the answer.

Lets take the equation:

a-b > a+b
now reverse the equality sign by multiplying with negative

a-b < -a-b
2a <0
which means a< 0

Please suggest if i am doing something wrong
Top 
adiagr
 Post subject: Re: MGMAT Problem
 Post Posted: Sun Jul 04, 2010 2:09 am 
Offline
Students


Posts: 89
dewanak wrote:
If a – b > a + b, where a and b are integers, which of the following must be true?

I. a < 0
II. b < 0
III. ab < 0

(1) I only

(2) II only

(3) I and II only

(4) I and III only

(5) II and III only

as per the MGMAT answer is (2)

I am ok with b<0 which comes directly from the equation but i say (3) is the answer.

Lets take the equation:

a-b > a+b
now reverse the equality sign by multiplying with negative

a-b < -a-b
2a <0
which means a< 0

Please suggest if i am doing something wrong


when you are multiplying with negative, it will be

(-a+b) < (-a-b) [you have to multiply left-hand side with (-) also]

b < -b
2b<0

b<0

Aditya
Top 
tim
 Post subject: Re: MGMAT Problem
 Post Posted: Thu Jul 08, 2010 2:14 pm 
Offline
ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
Thank you, adiagr..

_________________
Tim Sanders
Manhattan GMAT Instructor
Top 
Jason.tuyj
 Post subject: Re: MGMAT Problem
 Post Posted: Sun Jun 05, 2011 10:05 am 
Offline
Forum Guests


Posts: 36
why multiply (-) on each side? why A is not the answer as well?
Top 
Jason.tuyj
 Post subject: Re: MGMAT Problem
 Post Posted: Sun Jun 05, 2011 10:06 am 
Offline
Forum Guests


Posts: 36
why multiply (-) on each side? why A is not the answer as well?
Top 
jnelson0612
 Post subject: Re: MGMAT Problem
 Post Posted: Thu Jun 23, 2011 8:22 pm 
Offline
ManhattanGMAT Staff


Posts: 1857
Jason.tuyj wrote:
why multiply (-) on each side? why A is not the answer as well?


Here's how I would work this problem:

a – b > a + b
Subtract a from both sides
-b>b

At this point, I would say wait! When b has a negative sign in front it must be greater than a regular b? b must be a negative number in that case. For example, if b=-4, then -(-4)>-4, because that is 4>-4. We don't know anything about the value of a, so the answer is II only.

_________________
Jamie Nelson
ManhattanGMAT Instructor
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 6 posts ] 





Who is online

Users browsing this forum: No registered users and 1 guest

 
 

 
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 post attachments in this forum

Search for:
Jump to: