It is currently 17 Oct 2017, 01:10

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
2 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: 570

Kudos [?]: 2658 [2], given: 220

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
2
This post received
KUDOS
Top Contributor
17
This post was
BOOKMARKED
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

67% (01:05) correct 33% (01:05) wrong based on 312 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

Kudos [?]: 2658 [2], given: 220

4 KUDOS received
Math Forum Moderator
User avatar
P
Joined: 13 Apr 2015
Posts: 1501

Kudos [?]: 1101 [4], given: 884

Location: India
Concentration: Strategy, General Management
WE: Information Technology (Consulting)
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
4
This post received
KUDOS
5
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

Kudos [?]: 1101 [4], given: 884

3 KUDOS received
Senior Manager
Senior Manager
avatar
S
Joined: 22 Aug 2013
Posts: 432

Kudos [?]: 130 [3], given: 260

Location: India
Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 12 Jun 2017, 21:10
3
This post received
KUDOS
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

Kudos [?]: 130 [3], given: 260

Manager
Manager
avatar
S
Joined: 21 Jun 2015
Posts: 53

Kudos [?]: 27 [0], given: 28

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

Kudos [?]: 27 [0], given: 28

Intern
Intern
avatar
Joined: 17 Sep 2017
Posts: 2

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

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

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

Intern
Intern
avatar
Joined: 08 Oct 2017
Posts: 1

Kudos [?]: 0 [0], given: 7

Re: For each student in a certain class, a teacher adjusted the student’s [#permalink]

Show Tags

New post 16 Oct 2017, 17:54
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.

Kudos [?]: 0 [0], given: 7

Re: For each student in a certain class, a teacher adjusted the student’s   [#permalink] 16 Oct 2017, 17:54
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  


cron

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®.