一道GMATPrep题（在汇总里米瞧见）

棱角优雅的冰

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

[此贴子已经被作者于2006-2-14 5:51:28编辑过]

棱角优雅的冰

现在我知道了，每个tank都拿走30%，说明standard deviation是10*0.7=7，所以是A

bigmachine

设问？

llxx1985cn

把老贴又顶出来了

有一个疑问：虽然都移走了30%，但是没说每个的是相等的阿？这样移走的数不一定相等啊？

[此贴子已经被作者于2007-8-26 18:34:23编辑过]

boshu2007

这提示高中数学II中统计初步里的公式应用吧？

用的是方差的线性特征：

D(aX+b) = a^2DX  (DX指X系列值的二阶矩或者说方差)

题说的SD是二阶矩DX的平方根，那么

SD(aX+b) = |a|SD(X)

我做PREP时这道题换了一组条件。

llxx1985cn

有点看不明白，偶的问题还是没有解决啊？

[此贴子已经被作者于2007-8-26 20:27:35编辑过]

llxx1985cn

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charismale

以下是引用llxx1985cn在2007-8-26 20:26:00的发言：

有点看不明白，偶的问题还是没有解决啊？

恕我直言，你的解答只能算是蒙对了。。。

llxx1985cn

我的什么解答啊？这题到底应该怎么考虑？？

llxx1985cn

up