During an experiment, some water was removed from each of the 6 water tanks. If the standard deviation of the volumes of

[BTGmoderatorDC]

During an experiment, some water was removed from each of the 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% 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.



OA A

Source: GMAT Prep

[Jay@ManhattanReview]

During an experiment, some water was removed from each of the 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% 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.

OA A

Source: GMAT Prep

Let's take each statement one by one.

(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.

Since 30% = 3/10 part of the water is removed, 7/10 part of the water in each tank is remaining.

So, the new volumes of water in each of the 6 tanks are multiplied by a factor of 7/10.

Note: If you were to multiply each value in a data set by a constant c, the standard deviation would also be multiplied by |c|.

Thus, since each of 6 values are multiplied by 7/10, SD would be 10*7/10 = 7 gallons. Sufficient.

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

Since we do not know how much water was removed, we cannot find out the value of the revised SD. Insufficient.

The correct answer: A

Hope this helps!

-Jay
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