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Standard Deviation - Water tank

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sreak1089: Master | Next Rank: 500 Posts; Posts: 379; Joined: Wed Jun 03, 2009 3:05 amFollowed by: 1 member; Thanked: 19 times; GMAT Score:690

by sreak1089 » Fri Aug 07, 2009 5:27 am

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?

1) For each tank, 30% of water at the beginning was removed
2) The average volume of water in the tanks at the end was 63 gallons

OA A

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Source: Beat The GMAT — Data Sufficiency | Fri Aug 07, 2009 5:27 am

sep2009: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Thu Jul 30, 2009 5:06 am

Is it B?

by sep2009 » Fri Aug 07, 2009 5:45 am

Is it B?:?:

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vinayakdl: Master | Next Rank: 500 Posts; Posts: 131; Joined: Wed May 06, 2009 1:01 pm; Location: Chicago; Thanked: 7 times

by vinayakdl » Fri Aug 07, 2009 5:55 am

Agree with A as answer.

1) when the numbers are all reduced by same percentage the SD is reduced by equivalent percentage.

2) any two set of numbers can have same average but different SD, hence not sufficient.

Good one!

Vinayak

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shahdevine: Master | Next Rank: 500 Posts; Posts: 197; Joined: Sun May 18, 2008 2:47 am; Thanked: 12 times

Re: Standard Deviation - Water tank

by shahdevine » Fri Aug 07, 2009 6:27 am

sreak1089 wrote: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?

1) For each tank, 30% of water at the beginning was removed
2) The average volume of water in the tanks at the end was 63 gallons

OA A

s1)
This is conceptual...they are saying standard deviation is 10 gallons. Since the same amount of water 30% was moved at the beginning the standard deviation remains unchanged at end. so Standard deviation is 10. sufficient.

s1) this tells us nothing about amount of water removed. we need to know that in order to calculate standard deviation. we need to know the exact amount of water taken from each tank as that number affects standard deviation. Having an average does not tell you how each tank was affected. insufficient.

A

you got this, man!

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