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题库 / Prep2007E1-DS-34
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.
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
一批数字按同百分比变化，前后标准差比例与这个百分比有关
一批数字按同数额变化，前后标准差不变
样本同时乘以或除以一个数,方差乘以或除以该数的平方
样本同时加上或减去一个数,方差不变
不好意思，上一个回答是Variance，SD是variance开根
new SD=1/n sum [30%Xi - 30%E(X)]^2，可以把30%提出来，所以第一个条件就够了
做对了但是复习知识点：standard deviation标准差 平方是方差 方差=[（x-平均数）^2+（y-平均数）^2]/2 方差是要除以n的
gmat不会考到方差吧？考到我就要糊了
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% change in each of the terms:
- the standard deviation changes according to the same change
- the arithmetic mean also is changed by the same %
Adding either positive or negative constant
-the standard deviation does not change
-the arithmetic mean is dislocated by the same constant