GMAT Changed on April 16th - Read about the latest changes here

It is currently 27 Apr 2018, 01:27

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

For each student in a certain class, a teacher adjusted the student’s

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

Hide Tags

Top Contributor
5 KUDOS received
Director
Director
User avatar
B
Status: I don't stop when I'm Tired,I stop when I'm done
Joined: 11 May 2014
Posts: 557
Location: Bangladesh
Concentration: Finance, Leadership
GPA: 2.81
WE: Business Development (Real Estate)
For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 12 Jun 2017, 15:23
5
This post received
KUDOS
Top Contributor
27
This post was
BOOKMARKED
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

68% (01:02) correct 32% (01:02) wrong based on 673 sessions

HideShow timer Statistics

For each student in a certain class, a teacher adjusted the student’s test score using the formula y = 0.8x + 20, where x is the student’s original test score and y is the student’s adjusted test score. If the standard deviation of the original test scores of the students in the class was 20, what was the standard deviation of the adjusted test scores of the students in the class?

A. 12
B. 16
C. 28
D. 36
E. 40

The Official Guide for GMAT Review 2018

Practice Question
Problem Solving
Question No.: 151
[Reveal] Spoiler: OA

_________________

Md. Abdur Rakib

Please Press +1 Kudos,If it helps
Sentence Correction-Collection of Ron Purewal's "elliptical construction/analogies" for SC Challenges

16 KUDOS received
SC Moderator
User avatar
P
Joined: 13 Apr 2015
Posts: 1621
Location: India
Concentration: Strategy, General Management
WE: Analyst (Retail)
GMAT ToolKit User Premium Member CAT Tests
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 12 Jun 2017, 20:48
16
This post received
KUDOS
6
This post was
BOOKMARKED
Addition of 20 doesn't change the standard deviation. However, multiplying the original score by 0.8 does change the standard deviation.

Standard deviation of original score = 20

Standard deviation of adjusted score = 0.8 * 20 = 16

Answer: B
6 KUDOS received
DS Forum Moderator
avatar
G
Joined: 22 Aug 2013
Posts: 1045
Location: India
Premium Member CAT Tests
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 12 Jun 2017, 21:10
6
This post received
KUDOS
3
This post was
BOOKMARKED
When all the terms of a set are increased/decreased by a certain percent, then the Std Deviation also increases/decreases by the same percent.
If all the terms of a set are increased/decreased by a certain constant, the Std Deviation does not change.

Here y = 0.8x + 20

So all the original scores are being multiplied by 0.8 (thus Std Dev will also be multiplied by 0.8), and then are increased by 20 (which will have NO further effect on Std Dev).

So the new Std Deviation = 0.8*20 = 16.

Hence B answer
Manager
Manager
avatar
S
Joined: 21 Jun 2015
Posts: 51
Location: India
Concentration: Finance, General Management
GMAT 1: 660 Q50 V30
GPA: 3.32
WE: Programming (Computer Software)
GMAT ToolKit User
Re: For each student in a certain class, a teac [#permalink]

Show Tags

New post 18 Sep 2017, 13:22
2
This post was
BOOKMARKED
amanvermagmat wrote:
When all the terms of a set are increased/decreased by a certain percent, then the Std Deviation also increases/decreases by the same percent.
If all the terms of a set are increased/decreased by a certain constant, the Std Deviation does not change.

Here y = 0.8x + 20

So all the original scores are being multiplied by 0.8 (thus Std Dev will also be multiplied by 0.8), and then are increased by 20 (which will have NO further effect on Std Dev).

So the new Std Deviation = 0.8*20 = 16.

Hence B answer


Standard deviation of adjusted score = 0.8 * 20 = 16
Intern
Intern
avatar
Joined: 17 Sep 2017
Posts: 2
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 19 Sep 2017, 15:14
x is the student's original score. So how can we define directly that X= std deviation ?




amanvermagmat wrote:
When all the terms of a set are increased/decreased by a certain percent, then the Std Deviation also increases/decreases by the same percent.
If all the terms of a set are increased/decreased by a certain constant, the Std Deviation does not change.

Here y = 0.8x + 20

So all the original scores are being multiplied by 0.8 (thus Std Dev will also be multiplied by 0.8), and then are increased by 20 (which will have NO further effect on Std Dev).

So the new Std Deviation = 0.8*20 = 16.

Hence B answer
1 KUDOS received
Intern
Intern
avatar
B
Joined: 08 Oct 2017
Posts: 1
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 16 Oct 2017, 17:54
1
This post received
KUDOS
gmat75016 wrote:
x is the student's original score. So how can we define directly that X= std deviation ?




amanvermagmat wrote:
When all the terms of a set are increased/decreased by a certain percent, then the Std Deviation also increases/decreases by the same percent.
If all the terms of a set are increased/decreased by a certain constant, the Std Deviation does not change.

Here y = 0.8x + 20

So all the original scores are being multiplied by 0.8 (thus Std Dev will also be multiplied by 0.8), and then are increased by 20 (which will have NO further effect on Std Dev).

So the new Std Deviation = 0.8*20 = 16.

Hence B answer


I actually took a way longer route... maybe it will help

If the teacher would change every single grade, he would use the same formula for every student and would get the standard deviation at the end.
So every score would change after going through the same formula.

for instance, let's say there were 3 students in the class and the grades were 20, 40 and 60.
if we substitute every x for the grade we will get:

1) grade 20
y = 0.8 x 20 + 20 = 36

2) grade 40
y = 0.8 x 40 + 20 = 52

3) grade 60
y = 0.8 x 60 + 20 = 68

The adjusted grades are then 36, 52, 68 , with new a standard deviation of 16

Before we add the "20" we will get 16, 32 and 48; with the same standard deviation - 16

So I guess that since the standard deviation is a way to measure the relation between the numbers, by multiplying it by the same variable (0.8) we can adjust right away for this new relation.
Expert Post
2 KUDOS received
Target Test Prep Representative
User avatar
G
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 2493
Location: United States (CA)
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 19 Oct 2017, 10:29
2
This post received
KUDOS
Expert's post
3
This post was
BOOKMARKED
AbdurRakib wrote:
For each student in a certain class, a teacher adjusted the student’s test score using the formula y = 0.8x + 20, where x is the student’s original test score and y is the student’s adjusted test score. If the standard deviation of the original test scores of the students in the class was 20, what was the standard deviation of the adjusted test scores of the students in the class?

A. 12
B. 16
C. 28
D. 36
E. 40


Let’s review two rules for the standard deviation: 1. Adding a constant to each value in a data set does not change the value of the standard deviation; 2. Multiplying each value in a data set by a constant also multiplies the standard deviation by that constant.

Thus, for y = 0.8x + 20, we see that adding 20 does not affect the value of the standard deviation. But multiplying each score by 0.8 means that the standard deviation of the entire data set is also multiplied by 0.8; thus, the standard deviation of the adjusted test scores will be 20 x 0.8 = 16.

Answer: B
_________________

Scott Woodbury-Stewart
Founder and CEO

GMAT Quant Self-Study Course
500+ lessons 3000+ practice problems 800+ HD solutions

Senior Manager
Senior Manager
User avatar
G
Joined: 09 Mar 2016
Posts: 450
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 02 Dec 2017, 06:32
ScottTargetTestPrep wrote:
AbdurRakib wrote:
For each student in a certain class, a teacher adjusted the student’s test score using the formula y = 0.8x + 20, where x is the student’s original test score and y is the student’s adjusted test score. If the standard deviation of the original test scores of the students in the class was 20, what was the standard deviation of the adjusted test scores of the students in the class?

A. 12
B. 16
C. 28
D. 36
E. 40


Let’s review two rules for the standard deviation: 1. Adding a constant to each value in a data set does not change the value of the standard deviation; 2. Multiplying each value in a data set by a constant also multiplies the standard deviation by that constant.

Thus, for y = 0.8x + 20, we see that adding 20 does not affect the value of the standard deviation. But multiplying each score by 0.8 means that the standard deviation of the entire data set is also multiplied by 0.8; thus, the standard deviation of the adjusted test scores will be 20 x 0.8 = 16.

Answer: B


why answer is B and not D ? id STD deviation is 20 hence by plugging it in the given formula y = 0.8 (20) + 20 ---> y = 16 +20 --> y = 36 no? :? :)
DS Forum Moderator
avatar
G
Joined: 22 Aug 2013
Posts: 1045
Location: India
Premium Member CAT Tests
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 02 Dec 2017, 11:11
dave13 wrote:
ScottTargetTestPrep wrote:
AbdurRakib wrote:
For each student in a certain class, a teacher adjusted the student’s test score using the formula y = 0.8x + 20, where x is the student’s original test score and y is the student’s adjusted test score. If the standard deviation of the original test scores of the students in the class was 20, what was the standard deviation of the adjusted test scores of the students in the class?

A. 12
B. 16
C. 28
D. 36
E. 40


Let’s review two rules for the standard deviation: 1. Adding a constant to each value in a data set does not change the value of the standard deviation; 2. Multiplying each value in a data set by a constant also multiplies the standard deviation by that constant.

Thus, for y = 0.8x + 20, we see that adding 20 does not affect the value of the standard deviation. But multiplying each score by 0.8 means that the standard deviation of the entire data set is also multiplied by 0.8; thus, the standard deviation of the adjusted test scores will be 20 x 0.8 = 16.

Answer: B


why answer is B and not D ? id STD deviation is 20 hence by plugging it in the given formula y = 0.8 (20) + 20 ---> y = 16 +20 --> y = 36 no? :? :)


Hi

y is not the formula for standard deviation. 'y' here refers to the adjusted value or the changed value of 'x', where 'x' was the original value. So basically teacher is multiplying each original value by 0.8, and then adding 20 so as to get a new value. We have to find standard deviation of this new set of values, where we are given the standard deviation of old set of values was 20.
Intern
Intern
User avatar
B
Joined: 26 Feb 2018
Posts: 26
WE: Sales (Internet and New Media)
CAT Tests
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 16 Mar 2018, 09:34
Made a blunder , by adding 16 + 20 to the actual standard deviation . Corrected it QA : 16 (ANSWER)
_________________

" Can't stop learning and failing"

Re: For each student in a certain class, a teacher adjusted the student’s   [#permalink] 16 Mar 2018, 09:34
Display posts from previous: Sort by

For each student in a certain class, a teacher adjusted the student’s

  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®.