I would like some advice on DS questions

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I would like some advice on DS questions

by chrvlachos » Sun Jul 26, 2015 11:55 am
Hello everyone,

I have been studying for the GMAT for a few months. My GMAT exam date is on 25th of August, 2015. From the very beginning, I had major issues with DS questions. I have improved over the last few weeks but they still remain a hurdle towards a good final grade.

My last CAT from GMATPrep was as follows:
650 Q43 (61%) V37 (81%)
The problem is that of the 18 wrong questions in Quant, 9 of them were DS questions and not necessarily of high level. More specifically, these were the DS questions I got wrong:
What would your advice be towards improving on DS questions?

1)If x and y are integers greater than 1, is x a multiple of y?

(1) 3y^2+7y=x
(2) x^2-x is a multiple of y


2)A contractor combined x tons of a gravel mixture that contained 10 percent gravel G, by weight, with y tons of a mixture that contained 2 percent gravel G, by weight, to produce z tons of a mixture that was 5 percent gravel G, by weight. What is the value of x ?

(1) y = 10

(2) z = 16


3)In the figure shown, what is the value of x?
Image

(1) The length of line segment QR is equal to the length of line segment RS

(2) The length of line segment ST is equal to the length of line segment TU


4)Janson's salary and Karen's salary were each p percent greater in 1998 than in 1995. What is the value of p?

(1) In 1995 Karen's salary was $2,000 greater than Jason's.
(2) In 1998 Karen's salary was $2,440 greater than Jason's.


5)If s and t are two different numbers on the number line, is s + t = 0 ?

(1) Distance between s and 0 is the same as distance between t and 0
(2) 0 is between s and t

6)If it took Carlos 1/2 hour to cycle from his house to the library yesterday, was the distance that he cycled greater than 6 miles? (Note: 1 mile=5280 feet.

1) The average speed at which Carlos cycled from his house to the library yesterday was greater than 16 feet per second.
2) The average speed at which Carlos cycled from his house to the library yesterday was greater than 18 feet per second.

7)For the students in class A, the range of their heights is r centimeters and the greatest height is g centimeters. For the students in class B, the range of their heights is s centimeters and the greatest height is h centimeters. Is the least height of the students in class A greater than the least height of the students in class B ?

(1) r < s
(2) g > h

8)Is x less than 20?

(1) The sum of x and y is less than 20.
(2) y is less than 20.

9)If the average (arithmetic mean) of four different numbers is 30, how many of the numbers are greater than 30?

(1) None of the four numbers is greater than 60.
(2) Two of the four numbers are 9 and 10, respectively.
Source: — Data Sufficiency |

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by Brent@GMATPrepNow » Sun Jul 26, 2015 1:04 pm
Please post only one question per thread. Otherwise things can become pretty complicated when there are discussions on multiple questions.

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by Brent@GMATPrepNow » Sun Jul 26, 2015 1:05 pm
chrvlachos wrote:Hello everyone,

I have been studying for the GMAT for a few months. My GMAT exam date is on 25th of August, 2015. From the very beginning, I had major issues with DS questions. I have improved over the last few weeks but they still remain a hurdle towards a good final grade.

Your situation is very common among people preparing for the GMAT. The truth of the matter is that 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 (like confusing the answer to sufficiency question to the answer to the target question), but it's easier to just direct you to our free set of 16 videos that cover everything you need to know to tackle DS questions: https://www.gmatprepnow.com/module/gmat-data-sufficiency

Cheers,
Brent
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by chrvlachos » Sun Jul 26, 2015 11:21 pm
The DS questions were posted here because i am seeking advice on where to focus my studying in concerning DS, not because I am looking for their solution.
Thank you very much for your advice Brent!

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by prachi18oct » Mon Jul 27, 2015 9:01 am
Please post the OAs of the above questions.

[spoiler]1.A
2.D
3.C
4.B
5.A
6.E
7.C
8.E
9.C[/spoiler]

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by [email protected] » Mon Jul 27, 2015 9:02 am
Hi chrvlachos,

I've sent you a Private Message.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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by Max@Math Revolution » Sat Aug 01, 2015 4:27 am
With DS questions, the best way to solve them is using the Variable Approach:

(DS) Number of variables:3 , question?

1) Number of equations:1

2) Number of equations:1

In this case we need 3 equations, but we only have 2, so the answer is E



DS) Number of variables:2 , question?

1) Number of equations:1

2) Number of equations:1

In this case we need 2 equations, so the answer is c



DS) Number of variables:1 , question?

1) Number of equations:1

2) Number of equations:1

In this case we need 1 equation, so the answer is likely D



Of course, the first thing you do is change the original condition.

In question 1, what they are asking is x=y(int)


In condition 1, since x=y(3y+7), we can answer "yes", and this condition is sufficient.

In condition 2, if x=2, y=2, then "yes", but if x=3, y=6, then "no", so this condition is not sufficient.
Therefore the answer is A, and the difficulty of this question is around 49 (out of 51)


With question 8), there are two variables (x, y), so we need two equations as well.
Taking a look at both conditions 1 and 2 together,
If y=-100, x=30 (no)
If y=-20, x=10 (yes)
So the answer is E.
We don't have to separately consider conditions 1 and 2, so we save a lot of time.


If you want to see more of Variable Approach like this, come to www.mathrevolution.com