Events & Promotions
|
|
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
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Nov 1912:30 PM EST
-01:30 PM EST
Learn how Keshav, a Chartered Accountant, scored an impressive 705 on GMAT in just 30 days with GMATWhiz's expert guidance. In this video, he shares preparation tips and strategies that worked for him, including the mock, time management, and more
Nov 2001:30 PM EST
-02:30 PM IST
Learn how Kamakshi achieved a GMAT 675 with an impressive 96th %ile in Data Insights. Discover the unique methods and exam strategies that helped her excel in DI along with other sections for a balanced and high score.
Nov 2211:00 AM IST
-01:00 PM IST
Do RC/MSR passages scare you? e-GMAT is conducting a masterclass to help you learn – Learn effective reading strategies Tackle difficult RC & MSR with confidence Excel in timed test environment- Nov 23
11:00 AM IST
-01:00 PM IST
Attend this free GMAT Algebra Webinar and learn how to master the most challenging Inequalities and Absolute Value problems with ease. 
Nov 2407:00 PM PST
-08:00 PM PST
Full-length FE mock with insightful analytics, weakness diagnosis, and video explanations!
Nov 2510:00 AM EST
-11:00 AM EST
Prefer video-based learning? The Target Test Prep OnDemand course is a one-of-a-kind video masterclass featuring 400 hours of lecture-style teaching by Scott Woodbury-Stewart, founder of Target Test Prep and one of the most accomplished GMAT instructors.
11 May 2015, 08:16
Kudos
Bookmarks
A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
95%
(hard)
Question Stats:
47% (02:29) correct
53%
(02:41)
wrong
based on 168
sessions
History
Date
Time
Result
Not Attempted Yet
Set S is composed of following real numbers {1/96 , 2/96 , .. , 96/96 } . Two numbers 'a' and 'b' are selected at random without replacement and are used to calculate result according to equation 2*a*b-a-b+1 and this result is put back into the set S .
This step is done repeatedly until all the numbers in the set are exhausted and set contains only 1 number. what is that last number ?
a) 1/2
b) 1/6
c) 1/8
d) 1/9
c) 1/12
Kudos for correct solution . Thanks
This step is done repeatedly until all the numbers in the set are exhausted and set contains only 1 number. what is that last number ?
a) 1/2
b) 1/6
c) 1/8
d) 1/9
c) 1/12
Kudos for correct solution . Thanks
11 May 2015, 21:35
Kudos
Bookmarks
This is a really tricky abstract question, and one that is probably best solved by backsolving. This kind of question is particularly dangerous because it is not only difficult to get correct, but it will bait test takers into spending far too long on it. Do not get "lost" in a problem, and know when to cut the cord on problems like this and make an educated guess. Veritas does a great job at teaching these high level strategies in their courses.
In terms of backsolving on this, I am using the term loosely because I mean plug in the answer choices and see what comes out of this formula...and if that means anything to you. For answer choice a (the correct answer), lets assign the value of 1/2 to the variable "a". The variable "b" will remain unknown. Plugging a = 1/2 into this equation, we have 2*1/2*b - 1/2 - b + 1...simplifying we get b - 1/2 - b + 1, or -1/2 + 1, or 1/2. This means that any time you use 1/2 for one of the variables in this equation, the equation will spit the result of 1/2 right back out. So when all is said and done, that is the only value that will remain.
Each of the other answer choices provide you a different output from input and keep a variable in the output (thus making the exact value impossible to know). Look at answer choice b for example. Doing the same thing, 2*1/6*b - 1/6 - b + 1 = -2/3b + 5/6. We of course cannot know what this is, as we do not know b, and it will change when b changes.
I hope this helps!
In terms of backsolving on this, I am using the term loosely because I mean plug in the answer choices and see what comes out of this formula...and if that means anything to you. For answer choice a (the correct answer), lets assign the value of 1/2 to the variable "a". The variable "b" will remain unknown. Plugging a = 1/2 into this equation, we have 2*1/2*b - 1/2 - b + 1...simplifying we get b - 1/2 - b + 1, or -1/2 + 1, or 1/2. This means that any time you use 1/2 for one of the variables in this equation, the equation will spit the result of 1/2 right back out. So when all is said and done, that is the only value that will remain.
Each of the other answer choices provide you a different output from input and keep a variable in the output (thus making the exact value impossible to know). Look at answer choice b for example. Doing the same thing, 2*1/6*b - 1/6 - b + 1 = -2/3b + 5/6. We of course cannot know what this is, as we do not know b, and it will change when b changes.
I hope this helps!
11 May 2015, 22:40
Kudos
Bookmarks
Lucky2783
Brilliant, Brandon!
I arrived at the answer after looking around a bit for a pattern.
Given is my train of thought for those who did not think of going the options route:
Set S: {1/96 , 2/96 , ... 95/96 , 96/96 }
Operation: 2ab - (a+b) + 1
My first thought was that the two numbers selected should be complementary such that they lead to a simple answer which would eventually lead to some kind of sequence.
It made sense to pick 1/96 and 95/96 so that a+b = 1. Similarly, 2/96 and 94/96 etc. We were left with 2ab from each operation. But the leftover values were too complicated to work with. Didn't work.
Next thought was using a number which simplified the operation such as: if one of the numbers picked was 1 (the last number 96/96), what would happen?
2*1*b - (1 + b) + 1 = b
Left with b - the other number picked up. Say b was 95/96. We put back 95/96 and now the set doesn't have 1 anymore. Back to square one.
The next logical choice was to try what happens when one of the numbers picked is 1/2 (the number 48/96) since that would take care of 2 of 2ab.
2*(1/2)b - 1/2 - b + 1 = 1/2
This was what worked! When one of the numbers picked was 1/2, it eventually led to 1/2. So every time, all you do is pick 48/96 with any other number and you will always put back 1/2. You keep doing this till you are left with just 1/2.
Answer (A)
