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If (a-b)/c<0, is a>b?

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Director
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If (a-b)/c<0, is a>b? [#permalink] New post 16 Mar 2013, 21:15
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A
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Difficulty:

  25% (medium)

Question Stats:

75% (01:57) correct 25% (01:29) wrong based on 51 sessions
If (a-b)/c<0, is a>b?

(1) c < 0
(2) a + b < 0
[Reveal] Spoiler: OA

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Last edited by Bunuel on 17 Mar 2013, 00:37, edited 1 time in total.
Renamed the topic and edited the question.
1 KUDOS received
Director
Director
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Joined: 03 Aug 2012
Posts: 895
Concentration: General Management, General Management
Schools: ISB '16
GMAT 1: 630 Q47 V29
GMAT 2: 680 Q50 V32
GPA: 3.7
Followers: 12

Kudos [?]: 189 [1] , given: 306

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Re: Inequalities [#permalink] New post 16 Mar 2013, 21:20
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(a-b)/c <0

=> c(a-b)/c^2 < 0
=> ac -bc <0
=> ac<bc

Statement (1) : Tells the sign of (C) means sufficient.

Statement (2) : a+b < 0

=> a < -b

a b a<-b Is a>b?
+ + Not Poss N/A
- - Yes No
- + Yes No
+ - Yes No

Why the answer is not (D).

Please tell what I am missing above

Rgds,
TGC
_________________

Rgds,
TGC!
_____________________________________________________________________
I Assisted You => KUDOS Please
_____________________________________________________________________________

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Re: If (a-b)/c<0, is a>b? [#permalink] New post 17 Mar 2013, 00:46
Before cross multiplying or multiplying numerator and denominator by c you should keep in mind that c should not be equal to 0
So which statement proves c won't be 0
(1) c < 0
This statement implies that c is not equal to 0
(2) a + b < 0
You aren't sure whether c would be 0 or not.
Hence, the answer should be A

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Re: If (a-b)/c<0, is a>b? [#permalink] New post 17 Mar 2013, 00:53
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If (a-b)/c<0, is a>b?

(1) c < 0. Multiply (a-b)/c<0 by negative c and flip the sign a-b>0 --> a>b. Sufficient.

(2) a + b < 0. The sum of two numbers is less than zero. Can we tell which of them is greater? (Can we tell whether a>b or a<b?) No, consider a=1, b=-2 and c=-1 AND a=-2, b=1 and c=1. Not sufficient.

Answer: A.
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Director
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Joined: 03 Aug 2012
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Re: If (a-b)/c<0, is a>b? [#permalink] New post 17 Mar 2013, 06:14
Bunuel wrote:
If (a-b)/c<0, is a>b?

(1) c < 0. Multiply (a-b)/c<0 by negative c and flip the sign a-b>0 --> a>b. Sufficient.

(2) a + b < 0. The sum of two numbers is less than zero. Can we tell which of them is greater? (Can we tell whether a>b or a<b?) No, consider a=1, b=-2 and c=-1 AND a=-2, b=1 and c=1. Not sufficient.

Answer: A.



What is wrong in the solution that I gave?

Rgds,
TGC
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TGC!
_____________________________________________________________________
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Expert Post
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Kudos [?]: 25267 [0], given: 2702

Re: Inequalities [#permalink] New post 17 Mar 2013, 06:54
Expert's post
targetgmatchotu wrote:
(a-b)/c <0

=> c(a-b)/c^2 < 0
=> ac -bc <0
=> ac<bc

Statement (1) : Tells the sign of (C) means sufficient.

Statement (2) : a+b < 0

=> a < -b

a b a<-b Is a>b?
+ + Not Poss N/A
- - Yes No
- + Yes No
+ - Yes No

Why the answer is not (D).

Please tell what I am missing above

Rgds,
TGC


Consider one of the cases in your approach above: if both a and b are negative, it's possible that a>b as well as a<b. For example, a=-1, b=-2, and c=-1 AND a=-2, b=-1, and c=1.

Hope it helps.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

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Director
Director
avatar
Joined: 03 Aug 2012
Posts: 895
Concentration: General Management, General Management
Schools: ISB '16
GMAT 1: 630 Q47 V29
GMAT 2: 680 Q50 V32
GPA: 3.7
Followers: 12

Kudos [?]: 189 [0], given: 306

Premium Member CAT Tests
Re: Inequalities [#permalink] New post 17 Mar 2013, 08:10
Bunuel wrote:
targetgmatchotu wrote:
(a-b)/c <0

=> c(a-b)/c^2 < 0
=> ac -bc <0
=> ac<bc

Statement (1) : Tells the sign of (C) means sufficient.

Statement (2) : a+b < 0

=> a < -b

a b a<-b Is a>b?
+ + Not Poss N/A
- - Yes No
- + Yes No
+ - Yes No

Why the answer is not (D).

Please tell what I am missing above

Rgds,
TGC


Consider one of the cases in your approach above: if both a and b are negative, it's possible that a>b as well as a<b. For example, a=-1, b=-2, and c=-1 AND a=-2, b=-1, and c=1.

Hope it helps.


Why to take c variable in picture .

Given that a<-b
I think appropriate would be
a = -1
b = -2 (here a>b still holding a<-b)

a = -2
b = -1 (here a <b still holding a<-b)

Hence,wrong
_________________

Rgds,
TGC!
_____________________________________________________________________
I Assisted You => KUDOS Please
_____________________________________________________________________________

1 KUDOS received
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Status: Final Lap Up!!!
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Joined: 21 Sep 2012
Posts: 1096
Location: India
GMAT 1: 410 Q35 V11
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GMAT 3: 630 Q45 V31
GPA: 3.84
WE: Engineering (Transportation)
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Re: Inequalities [#permalink] New post 17 Mar 2013, 08:38
1
This post received
KUDOS
targetgmatchotu wrote:
Bunuel wrote:
targetgmatchotu wrote:
(a-b)/c <0

=> c(a-b)/c^2 < 0
=> ac -bc <0
=> ac<bc

Statement (1) : Tells the sign of (C) means sufficient.

Statement (2) : a+b < 0

=> a < -b

a b a<-b Is a>b?
+ + Not Poss N/A
- - Yes No
- + Yes No
+ - Yes No

Why the answer is not (D).

Please tell what I am missing above

Rgds,
TGC


Consider one of the cases in your approach above: if both a and b are negative, it's possible that a>b as well as a<b. For example, a=-1, b=-2, and c=-1 AND a=-2, b=-1, and c=1.

Hope it helps.


Why to take c variable in picture .

Given that a<-b
I think appropriate would be
a = -1
b = -2 (here a>b still holding a<-b)

a = -2
b = -1 (here a <b still holding a<-b)

Hence,wrong


Hi TGC
good to see you back on the forums..........people gather courage from life's storms.

I think the process you suggested is correct and has got no flaws in it....its concise...i do not find any use of third variable here.

Archit
Re: Inequalities   [#permalink] 17 Mar 2013, 08:38
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