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water in the tanks

by sanju09 » Thu Mar 22, 2012 1:49 am
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 (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.
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by killer1387 » Thu Mar 22, 2012 2:10 am
should be A
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by Anurag@Gurome » Thu Mar 22, 2012 2:16 am
sanju09 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 (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.
(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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by GMATGuruNY » Thu Mar 22, 2012 3:26 am
sanju09 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 (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.
If every element in a set is increased or decreased by the same CONSTANT, then the standard deviation DOES NOT CHANGE.

If every element in a set is increased or decreased by the SAME PERCENTAGE, then the standard deviation WILL CHANGE BY THE SAME PERCENTAGE.

Statement 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.
Since every value was decreased by 30%, the SD at the end of the experiment was 70% of the old SD:
.7(10) = 7.
SUFFICIENT.

Statement 2: The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.
No way to determine the SD.
INSUFFICIENT.

The correct answer is A.
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