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Tank

by piyush_nitt » Tue May 05, 2009 2:09 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

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Re: Tank

by Vemuri » Tue May 05, 2009 8:24 pm
piyush_nitt 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?
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

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IMO E. Combing both the statements, we get to know that the avg volume in the water tanks before was 210 gallons. But, we do not know the spread & so calculating the SD for the new volume is not possible.
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by scoobydooby » Wed May 06, 2009 9:12 am
would go for A

1) after 30% water removed from each tank, the final volumes of the 6 tanks are 0.7v1, 0.7v2, 0.7v3....0.7v6. before removal the volumes were v1, v2, v3...v6
so SD after removal becomes 0.7*10=7 (SD changes each item is multiplied/divided by the same constant)
sufficient

2) average of ending volumes is 63g
we do not know the individual volumes, we only have mean.
not sufficient

hence, A
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by Uri » Wed May 06, 2009 10:54 am
vote for (A).
if same percentage of water is removed from each of the tanks, then the std deviation will remain same. so, (1) is sufficient.
until and unless we know the exact volume of water in each of the tanks. so, (2) is not sufficient.
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