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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.
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Re: m11#9 Official Explanation is incomplete ?? [#permalink]
 01 Nov 2008, 11:51
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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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Re: m11#9 Official Explanation is incomplete ?? [#permalink]
01 Nov 2008, 12:53
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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Re: m11#9 Official Explanation is incomplete ?? [#permalink]
03 Nov 2008, 08:51
Yes, the part of the Official Explanation is missing. We'll edit it asap. +1. 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)) http://gmatclub.com/tests/m11#expl9I think the explanation is missing/incomplete. Please help me through this problem. _________________
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ans : E but approach of scthakur is great i got the ans by plug-in which take time thanks dear
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I also agree with E for the same reason as mentioned above
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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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E, addition or subtraction of Same constant term does not change standard deviation of the numbers
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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.
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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. 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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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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