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gmat club DS

by bblast » Wed Jul 13, 2011 10:54 pm
What is the median of set s = (a-b,b-a,a+b) ?

1:The mean of set S is equal to a+b
2:The range of set S is equal to 2b



A

please answer my query as well.

stat 1 is pretty straight:

stat-2.
Since I was able to determine that statement 2 says that the median is b-a, I considered it sufficient.

Official answer says since we cannot determine the value hence 2 is insuff.

Where has it been worded we need to find a value of the median ?

This is similar to saying- Which of the two numbers is greater a or b ?

statement 1. b = a.

Is'nt the above statement sufficient ???

On similar lines should statement 2 not be sufficient since we have a definite algebraic value
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by amit2k9 » Wed Jul 13, 2011 11:09 pm
a a+b/3 = a+b where a=-b

hence -2b,0,2b will always give 0 as the median. sufficient.

b means b-a and a+b can either be min or max values.

thus a-b is the median.since we don't know a-b = some value.Not sufficient.

A it is.
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by Ian Stewart » Thu Jul 14, 2011 9:50 am
bblast wrote:

Where has it been worded we need to find a value of the median ?
If you read the instructions given for DS on the GMAT, you'll see:

NOTE: In data sufficiency problems that ask for the value of a quantity, the data given in the statements are sufficient only when it is possible to determine exactly one numerical value for the quantity.

So if a DS question asks for the median of a set, you only have sufficient information if you can get a single numerical value for the median.
bblast wrote:
This is similar to saying- Which of the two numbers is greater a or b ?

statement 1. b = a.

Is'nt the above statement sufficient ???

On similar lines should statement 2 not be sufficient since we have a definite algebraic value
No, it is not the same type of question. The question you are asking does not ask for a value; it only asks whether a > b is true. You can answer that question without knowing the values of a or b.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by bblast » Thu Jul 14, 2011 10:17 am
Thanks Ian, I read the other discussion on this question between Bunuel and yourself, was very helpful.
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