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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#9Source: GMAT Club Tests  hardest GMAT questions



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01 Nov 2008, 10:51
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).



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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.



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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.



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29 Dec 2010, 05:38
E, addition or subtraction of Same constant term does not change standard deviation of the numbers



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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.



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26 Dec 2012, 05:12
amitdgr wrote: Which of the following sets has the same standard deviation as set (a, b, c)? (C) 2008 GMAT Club  m11#9 * \((ab, b^2, cb)\) * \((2a, b + a, c + b)\) * \((0, b + a, c  a)\) * \((ab, bc, ac)\) * \((ab + c, a(1 + b), b(1+a))\) Source: GMAT Club Tests  hardest GMAT questions http://gmatclub.com/tests/m11#expl9I 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. Answer: E.
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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.



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28 Oct 2013, 06:22
Awesome and crisp approach by scthakur and Bunuel..Great work



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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!










