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Kaplan math workbook simple averages question

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by a380038 » Mon Mar 12, 2012 3:54 pm
in the seventh edition Kaplan GMAT math workbook, I'm having trouble understanding the explanation of a DS proble.
#12 on the data sufficiency test one:

Stem: Does the average of a, b & c equal c?
--As I look at the stem I assume if a+b+c/3 does not = c then the answer is no.

1) c-a = c + b
So c= a+b+c
This must not be true right? The book does not address this approach. What am I missing?

2) c = 0
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Source: — Data Sufficiency |
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by Jim@StratusPrep » Mon Mar 12, 2012 6:55 pm
Statement 1 get's simplified to a = -b, this is Insufficient because you know nothing about c

2) gives you nothing about a and b

Together, they give you an average of 0.

C
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by [email protected] » Tue Mar 13, 2012 5:02 am
Yes the answer has to be C.

Option A leads to a conclusion where the requirement becomes something like:

c = c/3

How can you solve this equation even if c was given to you...

as c itself can never be equal to the division of its own number...

hence C:

Hope this helps...
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by vinaypat » Wed Mar 14, 2012 4:54 pm
1) c-a = c + b

i.e. -a = b

so (a+b+c)/3 = c/3 (substituting from above) is not equal to c unless c is 0 so insufficient

2) c = 0 No information about a and b so insufficient

So answer is C
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