Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 06 Oct 2015, 16:21

### 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

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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# m11#9

 Question banks Downloads My Bookmarks Reviews Important topics
Author Message
VP
Joined: 30 Jun 2008
Posts: 1047
Followers: 12

Kudos [?]: 404 [2] , given: 1

m11#9 [#permalink]  01 Nov 2008, 07:23
2
KUDOS
2
This post was
BOOKMARKED
Which of the following sets must have the same standard deviation as set {a, b, c}?

A. {ab, b^2, cb}
B. {2a, b + a, c + b}
C. {0, b + a, c - a}
D. {ab, bc, ac}
E. {ab + c, a(1 + b), b(1+a)}

(C) 2008 GMAT Club - m11#9

Source: GMAT Club Tests - hardest GMAT questions
_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

 Kaplan GMAT Prep Discount Codes Knewton GMAT Discount Codes Veritas Prep GMAT Discount Codes
SVP
Joined: 17 Jun 2008
Posts: 1570
Followers: 12

Kudos [?]: 213 [17] , given: 0

Re: m11#9 [#permalink]  01 Nov 2008, 10:51
17
KUDOS
The standard deviation of a set does not change if a constant is added to all the members.

Thus, standard deviation of (a,b,c) will be the same as of (a+ab, b+ab, c+ab).

And, option E is the same as (a+ab, b+ab, c+ab).
SVP
Joined: 29 Aug 2007
Posts: 2493
Followers: 62

Kudos [?]: 605 [0], given: 19

Re: m11#9 [#permalink]  01 Nov 2008, 11:53
scthakur wrote:
The standard deviation of a set does not change if a constant is added to all the members.

Thus, standard deviation of (a,b,c) will be the same as of (a+ab, b+ab, c+ab).

And, option E is the same as (a+ab, b+ab, c+ab).

Beautiful approach by scthakur. Thats the best approach to this question. +1.

SD of a, b and c and (a+x), (b+x) and (c + x) is the same.
Trying to find exactly what is the SD of a, b and c and the same of each of the options in the question doesnot help solve this question. What helps is understanding the question.
_________________
Joined: 31 Dec 1969
Location: India
Concentration: Strategy, Operations
GMAT 1: 710 Q49 V0
GMAT 2: 740 Q40 V50
GMAT 3: 700 Q48 V38
GMAT 4: 710 Q45 V41
GPA: 3.3
WE: Sales (Investment Banking)
Followers: 0

Kudos [?]: 115 [0], given: 90628

Re: m11#9 [#permalink]  22 Dec 2010, 06:02
My approach was actually using real numbers such as a=2, b=3 and c=4. Though abit lengthy it worked since I applied the rule, the less spread out my answers were the closer my answer was making it E. Thanx now I know another rule;The standard deviation of a set does not change if a constant is added to all the members.
Intern
Joined: 28 Jul 2010
Posts: 9
Followers: 0

Kudos [?]: 1 [0], given: 1

Re: m11#9 [#permalink]  29 Dec 2010, 05:38
E, addition or subtraction of Same constant term does not change standard deviation of the numbers
Manager
Joined: 21 Nov 2010
Posts: 133
Followers: 0

Kudos [?]: 5 [0], given: 12

Re: m11#9 [#permalink]  27 Dec 2011, 21:07
Standard Deviation is the spread of numbers. question is asking which spread of letters equals a, b, c.

I picked numbers 2, 4, 6 for a, b, c.
Plugged in to find another set that has the same SD of 2. E is the only one that worked.

------
Please give me kudos if my post helps you.
Math Expert
Joined: 02 Sep 2009
Posts: 29750
Followers: 4894

Kudos [?]: 53367 [1] , given: 8155

Re: m11#9 [#permalink]  26 Dec 2012, 05:12
1
KUDOS
Expert's post
amitdgr wrote:
Which of the following sets has the same standard deviation as set (a, b, c)?

(C) 2008 GMAT Club - m11#9

* $$(ab, b^2, cb)$$
* $$(2a, b + a, c + b)$$
* $$(0, b + a, c - a)$$
* $$(ab, bc, ac)$$
* $$(ab + c, a(1 + b), b(1+a))$$

[Reveal] Spoiler: OA
E

Source: GMAT Club Tests - hardest GMAT questions

http://gmatclub.com/tests/m11#expl9

I think the explanation is missing/incomplete. Please help me through this problem.

Which of the following sets must have the same standard deviation as set {a, b, c}?

A. {ab, b^2, cb}
B. {2a, b + a, c + b}
C. {0, b + a, c - a}
D. {ab, bc, ac}
E. {ab + c, a(1 + b), b(1+a)}

If we add or subtract a constant to each term in a set the standard deviation will not change.

Notice that set {(ab + c, a(1 + b), b(1+a)}={c+ab, a+ab, b+ab}, so this set is obtained by adding some number ab to each term of set {a, b, c}, which means that those sets must have the same standard deviation.

_________________
Manager
Joined: 13 Feb 2012
Posts: 147
Location: Italy
Concentration: General Management, Entrepreneurship
GMAT 1: 560 Q36 V34
GPA: 3.1
WE: Sales (Transportation)
Followers: 4

Kudos [?]: 4 [0], given: 85

Re: m11#9 [#permalink]  19 Jan 2013, 06:41
Plugging numbers in it's not so time wasting, even though it is prone to errors.

I put a=1, b=2, c=3 with a S.D of +/- 1

A = 2,4,6
B = 2,3,5
C = 0,3,2
D = 2,6,3
E = 5,3,4

E is the only set that has its numbers spread one integer apart.
_________________

"The Burnout" - My Debrief

Kudos if I helped you

Andy

Intern
Status: At the end all are winners, Some just take a little more time to win.
Joined: 08 Oct 2013
Posts: 23
Location: India
Concentration: Finance, Accounting
GMAT Date: 11-20-2013
GPA: 3.97
WE: Consulting (Computer Software)
Followers: 0

Kudos [?]: 7 [1] , given: 45

Re: m11#9 [#permalink]  28 Oct 2013, 06:22
1
KUDOS
Awesome and crisp approach by scthakur and Bunuel..Great work
Current Student
Joined: 05 Dec 2013
Posts: 16
Followers: 0

Kudos [?]: 8 [0], given: 1

Re: m11#9 [#permalink]  20 Dec 2013, 18:33
I took a similar, although longer, approach to solving this problem as the person above me. Immediately understanding that this problem was evaluating the spread, I relied on the use of "plugging" numbers in for a,b,c (1,2,3) and then looked for a similar spread amongst the answer choices.

Having read and followed the Manhattan Advanced Quant books, I first started with E and realized that this is the right answer --> matches to my "target"

Would of been even quicker if I would of realized that "ab" is consistent, a constant, throughout the 3 terms; and adding a constant to the terms does not alter the spread. Thanks for the clarification on this one guys!
Re: m11#9   [#permalink] 20 Dec 2013, 18:33
Display posts from previous: Sort by

# m11#9

 Question banks Downloads My Bookmarks Reviews Important topics

Moderators: WoundedTiger, Bunuel

 Powered by phpBB © phpBB Group and phpBB SEO 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®.