[求助]prep的两道数学题

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.

Answer：A

这题有人解释过说平均数是原来的0.7，然后标准方差就不变。实在想不通。标准方差不是每项还要减去平均值再平方吗？标准方差应该变好多吧。

2.x的四次方加y的四次方=100.问the greatest possible value of x is between?

0~3    3~6    6~9    9~12   12~15

答案B，我选A。

选B的话，x取4，4的四次方都大于100了，这怎么可能呢？

请教啊！！！

第一题：设Xn为第n个蓄水池蓄的水，则

standard deviation = （X1-(X1+X2+X3+X4+X5+X6)/6)^2

+ （X2-(X1+X2+X3+X4+X5+X6)/6)^2

+ （X3-(X1+X2+X3+X4+X5+X6)/6)^2

+ （X4-(X1+X2+X3+X4+X5+X6)/6)^2

+ （X5-(X1+X2+X3+X4+X5+X6)/6)^2

+ （X6-(X1+X2+X3+X4+X5+X6)/6)^2

根据第一个条件，可以提出公共系数0.7，则

啊！原来是说最大值在哪个范围内啊！郁闷，题目意思理解错了。谢谢了！

还有，哪个说数学简单的！！！我就有不少题不会做，尤其是考试时时间还那么紧张！！！

嗯~ 我也是做CXD的,不过大家都说他的简单

做PREP我也错很多

一战时数学没怎么准备就去了.  才拿了38分, 简直是惨不忍睹..