During an experiment, some water was removed from each of 6 : DS Archive
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# During an experiment, some water was removed from each of 6

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During an experiment, some water was removed from each of 6 [#permalink]

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10 Dec 2007, 21:23
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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 standard deviation of the volumes of water in the tanks at the end of the experiment?

1. For each tank, 30 percent 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.
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11 Dec 2007, 00:04
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 standard deviation of the volumes of water in the tanks at the end of the experiment?

1. For each tank, 30 percent 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.

A. adding or taking out the same amount of water doesnot change the SD.
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11 Dec 2007, 01:16
Hi,
IT is NOT the same amount it is 30%, which means different amounts from different volumes.
think it is C
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11 Dec 2007, 02:15
I think it's A.

You won't have the same standard deviation at the end, but changing each data item by the same proportion will mean that you change the standard deviation by that proportion also. So the SD will be 30% less than the original SD.
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12 Dec 2007, 08:02
GK_Gmat wrote:
Fistail wrote:
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 standard deviation of the volumes of water in the tanks at the end of the experiment?

1. For each tank, 30 percent 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.

A. adding or taking out the same amount of water doesnot change the SD.

A

Agree w/ the explanation.

Guys, I think it does. it follows from the SD formula. SD numbers will not be proportionate because it is a matter of scale. In other words, with smaller mean the difference around it will be smaller. And you square the differences and then sum them and (!) divide by the same number which is 6.

i tried to figure out if both statements combined are sufficient. but thus far I think it is E. Again, it could be C (but not A)

It is an interesting question indeed.
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12 Dec 2007, 10:47
Another A, average doesnot give anth.

Ans: A
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12 Dec 2007, 23:44
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 standard deviation of the volumes of water in the tanks at the end of the experiment?

1. For each tank, 30 percent 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.

A but for different reasons.

1. adding or deducting same amount from the original data doesnot change the SD. in this case the SD is 10.
2. adding or deducting same % amount from the original data changes the SD by the same %. in this case, the sd is now 10(1-0.3) = 7
Re: DS: standard deviation   [#permalink] 12 Dec 2007, 23:44
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