急！求两道数学的答案，PREP里的，哭啊。。

debralee

（1）

70， 75， 80， 85， 90， 105， 105， 130， 130，130，

The list shown consists of the times, in seconds, that it took each of 10 school children to run a distance of 400 meters. if the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

A. 1  B. 2  C. 3  D. 4 E. 5

答案：B

没读懂。有关方差的概念都忘了，不会啊。。。那位NN推荐点相关资料看看。谢了。

（2）

下面这道题压根儿没读懂！

Alice's take-home pay last year was same each month, and she saved the same fraction of her take-home pay each month. The total amount of money that she had saved at the end of the year was 3 times the amount of that portion of her monthly take-home pay that she did NOT save. if all the money that she saved last year was from her take-home pay, what fraction of her take-home pay did she save each month?

 A. 1/2  B. 1/3  C. 1/4  D. 1/5  E. 1/6

答案：D

 

 

joshsongdk

1，先求MEAN， 再用MEAN减22.4.然后看多少小于这个数。得出答案是77.6，取78，得到2个数。70  75

[此贴子已经被作者于2008-6-9 20:58:24编辑过]

jetking

mean=100.此数以下超过一个方差22.4的，100-22.4=77.6.就是比77.6小的数有几个？70和75.

2,设每个月存的比例为x，不存的比例则为1-x；设每个月的take-home pay为S；则全年存的钱为12*x*S=3*（1-x）*S。可以推出来x=1/5。

joshsongdk

2：设每个月存S， 赚P

那么 12S＝3（P-S） 15S＝3P  得到1/5

mswlove

how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

这句话能麻烦解释一下吗，真的不懂啊，谢谢了

mswlove

恩，我懂了，意思是这10个数当中在平均数以下的数里，和平均数之间的差距大于一个标准差，也就是22.4.

debralee

多谢多谢！！