问一道Prep的模考数学题

cindynxw

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.

答案是，Statement (1) alone is sufficient.

请问，这个standard deviation 到底怎么算的？这道题是什么意思？谢谢各位！

falconcn

这里有公式：http://en.wikipedia.org/wiki/Standard_deviation；

standard deviation中文叫标准方差，它是用来描述数据离散程度的；
只要知道了每个数，以及平均数，就可以来算了；
条件（1）：由于该操作去掉了30%的水，所以每罐水的离散程度可以得知（或者带入公式，可以知道新的标准方差是原来的0.7）
条件（2）：由于不知道每个罐子里的水的情况，所以离散程度无从得知，所以不可以；

具体可以看看上面的链接；或者我记得GMATPrep的tutorial里面有，你可以看看。