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Master | Next Rank: 500 Posts
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by shibal » Sun Jun 28, 2009 2:25 pm
Some water was removed from each of 6 tanks. If standard deviation of the volumes of water at the beginning was 10 gallons, what was the standard deviation of the volumes at the end?
a. For each tank, 30% of water at the beginning was removed
b. The average volume of water in the tanks at the end was 63 gallons

oa A
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Source: — Data Sufficiency |
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by ssmiles08 » Sun Jun 28, 2009 3:08 pm
1) standard deviation decreases by 30%. so sufficient.

2) Insufficient b/c we don't know how much water may be taken out of EACH tank, since the standard deviation is really based on difference b/w the mean and each point away from the mean.

(A)
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by aj5105 » Sun Jun 28, 2009 8:52 pm
Check this link, Shibal.

https://www.beatthegmat.com/gmat-prep-qu ... 34898.html
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by electrico » Mon Jun 29, 2009 12:26 am
1. Since, there is uniform change in the data, SD will not change.

So, A is sufficient.

2. It just give the avg volume.,..no info on the actual data/SD...

not enough..


So, Answer is A.
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by wetandfire » Wed Jul 01, 2009 12:04 pm
I am on the bandwagon that the SD will change.

If 1 tank is 10, and the other is 20 and the other is 30, there is our 10 SD. If we take 30% away from all 3 we will be left with
10 - 3 = 7, 20 - 6 = 14, and 30 - 9 = 21. Our standard deviation is now 7.
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