Standard Deviation Problem;HELP GMAT Tomorrow

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soni_pallavi: Master | Next Rank: 500 Posts; Posts: 107; Joined: Tue Jun 05, 2012 3:25 amFollowed by: 13 members; Thanked: 1 times

by soni_pallavi » Sat Nov 24, 2012 12:03 am

Q1) During an experiment,some water was removed from each of 6 water tanks.If the Standard Deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons,what was the SD of the volumes of water in the tanks at the end of the experiment?

1)For each tank,30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

2)The average volume of water in the tanks at the end of the experiment was 63 gallons.

Thanks

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Source: Beat The GMAT — Data Sufficiency | Sat Nov 24, 2012 12:03 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sat Nov 24, 2012 3:29 am

soni_pallavi wrote:Q1) During an experiment,some water was removed from each of 6 water tanks.If the Standard Deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons,what was the SD of the volumes of water in the tanks at the end of the experiment?

1)For each tank,30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.

2)The average volume of water in the tanks at the end of the experiment was 63 gallons.

Thanks

(1) It is given that 30% of the volume of water is removed from all the tanks so standard deviation remains same; SUFFICIENT.

(2) The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons does not help in finding the standard deviation of the volumes of water in the tanks at the end of the experiment; NOT sufficient.

The correct answer is A.

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mariofelixpasku: Senior | Next Rank: 100 Posts; Posts: 41; Joined: Sun Sep 23, 2012 4:26 am; Thanked: 2 times

by mariofelixpasku » Thu Mar 21, 2013 2:22 pm

regarding statement 1.

shouldnt the stdev decrease because we multiply each factor by 0.7 ? hence the std. changes by that factor ?

thanks in advance

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