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DS Geometry

This topic has 5 expert replies and 2 member replies
scottchapman Junior | Next Rank: 30 Posts Default Avatar
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Target GMAT Score:
700

DS Geometry

Post Thu Feb 05, 2015 11:44 am
If L & W represent the length and width, respectively, of the rectangle above, what is the perimeter?

1). 2L+W=40
2). L+W=25

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Post Thu Feb 05, 2015 5:32 pm
scottchapman wrote:
Thank you all for your responses. I struggle with DS and I think part of the reason for it is not rewording the question effectively. It makes sense after I see you guys go through the steps, but for whatever reason, I continue to make the same errors. I'm assuming I just need more practice...
EVERYONE struggles with Data Sufficiency (DS) questions at first. Keep in mind that this question type is unique to the GMAT, so it's totally foreign territory.

It just takes time for the concepts and strategies to become secondary, at which point you may come to find that DS questions are actually easier than Problem Solving questions.

I could start listing dozens of DS strategies and common mistakes, but it's easier to just direct you to our free set of videos that cover everything you need to know to tackle DS questions: http://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent

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Top Reply
Post Sat Feb 07, 2015 4:43 am
scottchapman wrote:
Thank you all for your responses. I struggle with DS and I think part of the reason for it is not rewording the question effectively. It makes sense after I see you guys go through the steps, but for whatever reason, I continue to make the same errors. I'm assuming I just need more practice...
The geometry problem above is a VALUE DS: a DS whose question stem asks for a specific value.
One way to evaluate the statements -- and to avoid falling for a trap -- is to TEST TWO CASES.
If the desired value is THE SAME in each case, the statement is SUFFICIENT.
If the desired value is NOT the same in each case, the statement is INSUFFICIENT.

Quote:
If L & W represent the length and width, respectively, of the rectangle above, what is the perimeter?

1). 2L + W = 40
2). L + W = 25
Statement 1: 2L + W = 40
Case 1: L=10, W=20, with the result that 2L + W = 2*10 + 20 = 40
The following rectangle is yielded:
ttp://postimg.org/image/di44anzmr/" target="_blank">

Case 2: L=5, W=30, with the result that 2L + W = 2*5 + 30 = 40
The following rectangle is yielded:
ttp://postimg.org/image/j2q8w5u77/" target="_blank">

Since the perimeter is NOT the same in each case, INSUFFICIENT.

Statement 2: L+W = 25
Case 3: L=10, W=15
The following rectangle is yielded:
ttp://postimg.org/image/5r6f0q219/" target="_blank">

Case 4: L=5, W=20
The following rectangle is yielded:
ttp://postimg.org/image/4sfusza2t/" target="_blank">

Since the perimeter is THE SAME in each case, SUFFICIENT.

The correct answer is B.

One advantage to this approach:
It does not require any special insight.

_________________
Mitch Hunt
GMAT Private Tutor
GMATGuruNY@gmail.com
If you find one of my posts helpful, please take a moment to click on the "Thank" icon.
Available for tutoring in NYC and long-distance.
For more information, please email me at GMATGuruNY@gmail.com.

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Post Thu Feb 05, 2015 5:32 pm
scottchapman wrote:
Thank you all for your responses. I struggle with DS and I think part of the reason for it is not rewording the question effectively. It makes sense after I see you guys go through the steps, but for whatever reason, I continue to make the same errors. I'm assuming I just need more practice...
EVERYONE struggles with Data Sufficiency (DS) questions at first. Keep in mind that this question type is unique to the GMAT, so it's totally foreign territory.

It just takes time for the concepts and strategies to become secondary, at which point you may come to find that DS questions are actually easier than Problem Solving questions.

I could start listing dozens of DS strategies and common mistakes, but it's easier to just direct you to our free set of videos that cover everything you need to know to tackle DS questions: http://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent

_________________
Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with Beat The GMAT's free 60-Day Study Guide

Check out the online reviews of our course
Come see all of our free resources

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Post Sat Feb 07, 2015 4:43 am
scottchapman wrote:
Thank you all for your responses. I struggle with DS and I think part of the reason for it is not rewording the question effectively. It makes sense after I see you guys go through the steps, but for whatever reason, I continue to make the same errors. I'm assuming I just need more practice...
The geometry problem above is a VALUE DS: a DS whose question stem asks for a specific value.
One way to evaluate the statements -- and to avoid falling for a trap -- is to TEST TWO CASES.
If the desired value is THE SAME in each case, the statement is SUFFICIENT.
If the desired value is NOT the same in each case, the statement is INSUFFICIENT.

Quote:
If L & W represent the length and width, respectively, of the rectangle above, what is the perimeter?

1). 2L + W = 40
2). L + W = 25
Statement 1: 2L + W = 40
Case 1: L=10, W=20, with the result that 2L + W = 2*10 + 20 = 40
The following rectangle is yielded:
ttp://postimg.org/image/di44anzmr/" target="_blank">

Case 2: L=5, W=30, with the result that 2L + W = 2*5 + 30 = 40
The following rectangle is yielded:
ttp://postimg.org/image/j2q8w5u77/" target="_blank">

Since the perimeter is NOT the same in each case, INSUFFICIENT.

Statement 2: L+W = 25
Case 3: L=10, W=15
The following rectangle is yielded:
ttp://postimg.org/image/5r6f0q219/" target="_blank">

Case 4: L=5, W=20
The following rectangle is yielded:
ttp://postimg.org/image/4sfusza2t/" target="_blank">

Since the perimeter is THE SAME in each case, SUFFICIENT.

The correct answer is B.

One advantage to this approach:
It does not require any special insight.

_________________
Mitch Hunt
GMAT Private Tutor
GMATGuruNY@gmail.com
If you find one of my posts helpful, please take a moment to click on the "Thank" icon.
Available for tutoring in NYC and long-distance.
For more information, please email me at GMATGuruNY@gmail.com.

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Post Mon Feb 09, 2015 12:43 am
scottchapman wrote:
Thank you all for your responses. I struggle with DS and I think part of the reason for it is not rewording the question effectively. It makes sense after I see you guys go through the steps, but for whatever reason, I continue to make the same errors. I'm assuming I just need more practice...
You'll definitely get there. Once you get the hang of the format, you may find yourself scoring higher on DS than on PS, even if the questions seem hard. DS is MUCH more guessable than PS, so you can get away with a lot of inspiring guesswork / trial and error / risk intelligence, as opposed to PS where you often need to know the math in question.

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scottchapman Junior | Next Rank: 30 Posts Default Avatar
Joined
30 Jun 2013
Posted:
18 messages
Target GMAT Score:
700
Post Thu Feb 05, 2015 4:57 pm
Thank you all for your responses. I struggle with DS and I think part of the reason for it is not rewording the question effectively. It makes sense after I see you guys go through the steps, but for whatever reason, I continue to make the same errors. I'm assuming I just need more practice...

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Top Member

Post Thu Feb 05, 2015 12:10 pm
Scott, this is a good example of a C trap question.

C trap questions are set up to get the test taker thinking that both statements are needed to get to the answer.

In this case, one could be thinking this way. "To calculate the perimeter, I need the length and width."

That thinking might even seem somehow confirmed after working on Statement 1, which allows for multiple values of L and W.

So then you might jump to Statement 2 and find that once again you can't determine the value of L or W.

At that point maybe you notice that you can subtract Statement 2 from Statement 1 and easily calculate the values of L and W. "Ahhhh," you think. "The answer is C."

The thing is that that combining of the statements is not necessary. You don't actually need the values of L and W because P = (L + W) + (L + W). So as soon as you have L + W = 25 you are set. So Statement 2 is sufficient and the answer is B.

If you didn't see that and instead chose C, then you fell into a C trap.

So there's one thing to be aware of in order to do better on DS.

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m.w.murray@hotmail.com
http://infinitemindprep.com/
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