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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]  12 Nov 2006, 08:26
1. During an experiment, some water was removed from each of 6 water tanks. If the standard deviation of the volume of water in the tanks at the beginning of the experiment was 10gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

a) For each task, 30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment
b) The average volume of water in the tanks at the end of the experiment was 63 gallons.
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The answer for this one is (a)...

could some one explain??
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a) For each task, 30% of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment
b) The average volume of water in the tanks at the end of the experiment was 63 gallons.

A => Say v1,v2,v3,v4,v5,v6,v7 be th initial volumes. The new volumes are .7v1,.7v2... .7v7. => the new mean is .7M, where M is the original Mean.
=> the SD is .7 times the old. Hence Sufficient.
B => Gives you new Mean but no info on the new xi. So insuff.

Hence A.

Hope this helps.
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