Last visit was: 14 Jul 2025, 14:24 It is currently 14 Jul 2025, 14:24
Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 14 Jul 2025
Posts: 102,570
Own Kudos:
741,403
 [9]
Given Kudos: 98,182
Products:
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 102,570
Kudos: 741,403
 [9]
Kudos
Add Kudos
9
Bookmarks
Bookmark this Post
User avatar
Kurtosis
User avatar
Current Student
Joined: 13 Apr 2015
Last visit: 10 Nov 2021
Posts: 1,400
Own Kudos:
4,980
 [1]
Given Kudos: 1,228
Location: India
Products:
Posts: 1,400
Kudos: 4,980
 [1]
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
Mo2men
Joined: 26 Mar 2013
Last visit: 09 May 2023
Posts: 2,443
Own Kudos:
Given Kudos: 641
Concentration: Operations, Strategy
Schools: Erasmus (II)
Products:
Schools: Erasmus (II)
Posts: 2,443
Kudos: 1,438
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
MathRevolution
User avatar
Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Last visit: 27 Sep 2022
Posts: 10,078
Own Kudos:
18,742
 [1]
Given Kudos: 4
GMAT 1: 760 Q51 V42
GPA: 3.82
Expert
Expert reply
GMAT 1: 760 Q51 V42
Posts: 10,078
Kudos: 18,742
 [1]
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

Is 13N a positive number?

(1) -21N is a negative number
(2) N^2 < 1


When you modify the original condition and the question, 13n>0? becomes n>0?. There are 1 variables(n), which should match with the number of equations. So you need 1 equation. For 1) 1 equation, for 2) 1 equation, which is likely to make D the answer.
For 1), divide -21n<0 with -21 and n>0 is yes, which is sufficient.
For 2), from -1<n<1, n=-0.1 is no but n=0.1 is yes, which is not sufficient.
Therefore, the answer is A.


 For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
User avatar
BrentGMATPrepNow
User avatar
Major Poster
Joined: 12 Sep 2015
Last visit: 13 May 2024
Posts: 6,755
Own Kudos:
34,082
 [3]
Given Kudos: 799
Location: Canada
Expert
Expert reply
Posts: 6,755
Kudos: 34,082
 [3]
3
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
Is 13N a positive number?

(1) -21N is a negative number
(2) N² < 1

Target question: Is 13N a positive number?

Statement 1: -21N is a negative number
We know that (NEGATIVE)(POSITIVE) = NEGATIVE
Since -21 is NEGATIVE, it must be the case that N is POSITIVE
If N is POSITIVE, then 13N is definitely a positive number
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: N² < 1
There are several values of N that satisfy statement 2. Here are two:
Case a: N = 1/2, in which case N² = (1/2)² = 1/4, which is less than 1. In this case 13N = (13)(1/2) = 6.5. So, 13N is POSITIVE
Case b: N = -1/2, in which case N² = (-1/2)² = 1/4, which is less than 1. In this case 13N = (13)(-1/2) = -6.5. So, 13N is NEGATIVE
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer:
Cheers,
Brent
User avatar
bumpbot
User avatar
Non-Human User
Joined: 09 Sep 2013
Last visit: 04 Jan 2021
Posts: 37,398
Own Kudos:
Posts: 37,398
Kudos: 1,013
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
Moderator:
Math Expert
102570 posts