一道DS题

piggycandys

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.


答案是A
我想请问一下按照A操作之后，标准差是多少啊？都移走了30%的水为什么标准差不是10呢？是不是因为没说那六个tank是一样的啊？
快考试了没解决的问题有点多。。谢谢各位了！！！

piggycandys

谢谢你哈！！！祝好运~~

quellekind

如果标准差还是10的话，肯定是每个tank里移掉了同样多的水；因为存在标准差，所以实验之前各个tank里的水是不一样多的，那么各自的30%也是不等的；按照A的操作之后，标准差应该是10×70%=7吧，我只选了个小例子实验了一下。希望是对的。