It is currently 20 Oct 2017, 05:58

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

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

Does rst = 1? (1) rs = –1 (2) rt = –1

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Expert Post
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 41890

Kudos [?]: 128966 [0], given: 12185

Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 28 Aug 2017, 04:02
00:00
A
B
C
D
E

Difficulty:

  45% (medium)

Question Stats:

77% (00:43) correct 23% (01:27) wrong based on 38 sessions

HideShow timer Statistics

Kudos [?]: 128966 [0], given: 12185

Director
Director
User avatar
G
Joined: 18 Aug 2016
Posts: 512

Kudos [?]: 140 [0], given: 123

GMAT 1: 630 Q47 V29
GMAT ToolKit User Premium Member Reviews Badge
Re: Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 28 Aug 2017, 04:11
Bunuel wrote:
Does rst = 1?

(1) rs = –1
(2) rt = –1


(1) no info abt t..not sufficient

(2) no info abt s...not sufficient

on combining
s=-1/r
t=-1/r
r*-1/r*-1/r = 1/r
1/r is not necessarily equal to 1
not sufficient
E
_________________

We must try to achieve the best within us

Thanks
Luckisnoexcuse

Kudos [?]: 140 [0], given: 123

Senior Manager
Senior Manager
User avatar
G
Joined: 02 Jul 2017
Posts: 252

Kudos [?]: 59 [0], given: 60

Location: India
GMAT ToolKit User CAT Tests
Re: Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 28 Aug 2017, 05:34
Ques : is rst=1

1. rs= -1 => No information. Not sufficient
2. rt= -1 => No information. Not sufficient

1+2
'rs= -1 and rt= -1;

we can only conclude that :
r= +ve and both s and t are -ve
s= -1/r and t= -1/r

OR

r= -ve and both s and t are +ve
s= -1/r and t= -1/r

so rst = r*(-1/r)*(-1/r) =1/r

Not sufficeint

Answer : E

Kudos [?]: 59 [0], given: 60

Manager
Manager
User avatar
B
Joined: 19 Jul 2017
Posts: 86

Kudos [?]: 10 [0], given: 85

Location: India
Concentration: General Management, Strategy
GPA: 3.5
Re: Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 28 Aug 2017, 05:39
(1) rs = –1. No info on t ; insuff
(2) rt = –1. No info on s; insuff

Combining r^2st=1 i.e; rst=1/r; no idea if r=-1 . So insufficient. E

Kudos [?]: 10 [0], given: 85

Intern
Intern
avatar
B
Joined: 29 Jul 2012
Posts: 17

Kudos [?]: 1 [0], given: 21

Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 28 Aug 2017, 12:28
It can be easily seen that both the statements can't answer the Q independently. So lets combine the two statements.

As per A: rs=-1 --> r=-1/s and As per B: rt=-1, substitute value of r --> (-1/s)*t=-1 --> t=s. No lets test the equations with some numbers.
Test 1: let t=s=1/2 ans r=-2, all equations are satisfied but rst is not equal to 1
Test 2: let t=s=1 and r=-1, all equations are satisfied and rst is equal to 1
Test 3: let t=s=1/2 ans r=-4, all equations are satisfied and rst is equal to 1

So it can't be uniquely established if rst=1 or not, hence, the answer is E.

Kudos [?]: 1 [0], given: 21

Expert Post
Math Revolution GMAT Instructor
User avatar
P
Joined: 16 Aug 2015
Posts: 4142

Kudos [?]: 2895 [0], given: 0

GPA: 3.82
Premium Member CAT Tests
Re: Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 28 Aug 2017, 20:31
Bunuel wrote:
Does rst = 1?

(1) rs = –1
(2) rt = –1


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.

There are 3 variables and 0 equation. Thus the answer E is most likely.

Consider both conditions together.
We have two kinds of solutions as follows.

r = 1, s = -1, t = -1. Then we have rst = 1.
r = -1, s = 1, t = 1. The we have rst = -1.
We don't have a unique solution.

Both conditions together are not sufficient.
The answer is E as expected.

For cases where we need 3 more equations, such as original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 80 % chance that E is the answer, while C has 15% chance and A, B or D has 5% chance. Since E is most likely to be the answer using 1) and 2) together according to DS definition. Obviously there may be cases where the answer is A, B, C or D.
_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
Find a 10% off coupon code for GMAT Club members.
“Receive 5 Math Questions & Solutions Daily”
Unlimited Access to over 120 free video lessons - try it yourself
See our Youtube demo

Kudos [?]: 2895 [0], given: 0

Manager
Manager
avatar
G
Joined: 06 Jan 2015
Posts: 208

Kudos [?]: 98 [0], given: 448

Location: India
Concentration: Operations, Finance
GPA: 3.35
WE: Information Technology (Computer Software)
Re: Does rst = 1? (1) rs = –1 (2) rt = –1 [#permalink]

Show Tags

New post 29 Aug 2017, 01:45
Similar question: https://gmatclub.com/forum/is-rst-90398.html
_________________

आत्मनॊ मोक्षार्थम् जगद्धिताय च

Kudos [?]: 98 [0], given: 448

Re: Does rst = 1? (1) rs = –1 (2) rt = –1   [#permalink] 29 Aug 2017, 01:45
Display posts from previous: Sort by

Does rst = 1? (1) rs = –1 (2) rt = –1

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.