急问PREP数学题~求NN解答！

sarah_kim · 发表于 2011-9-25 15:22:38

106.11122-!-item-!-187;#058&007588

If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than

2.5 standard deviations from the mean?

(A) 12.0

(B) 13.5

(C) 17.0

(D) 23.5

(E) 26.5

------------------------------------------------------------------------------------------------------------------------------------

【答案】A

62.7564-!-item-!-187;#058&005577

Alice
's take-home pay last year was the 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

没看懂具体意思。。

多谢了！！

fdunan · 发表于 2011-9-25 15:28:53

1：
该数>20+3*2.5 or 该数<20-3*2.5
即该数>27.5 or 该数<12.5
A满足

sarah_kim · 发表于 2011-9-25 15:43:23

为什么要*3呢？？

oxford2013 · 发表于 2011-9-25 16:09:55

第二题，设每月的homepage数为M，save下来的数为S，

则not save的数量（每月）=M-S 且一年存下来的数=12*S ，

根据原题所说，一年存下来的钱是每月没有存的钱的三倍，可列式子12*S=3(M-S)

化解得，M=5S,就oK啦~~

fj626888 · 发表于 2011-9-25 17:15:22

嗯，我也求解第一个题目的解释，不是很明白……

fdunan · 发表于 2011-9-25 17:21:19

3是标准差，原问题“more than 2.5 standard deviations from the mean”离均值大于2.5个标准差

为什么要*3呢？？

-- by 会员 sarah_kim (2011/9/25 15:43:23)

fj626888 · 发表于 2011-9-25 17:26:25

OMG！原来是这个意思……我理解错了，还以为哪个数是离20的标准差是2.5的数……汗！

多谢啦！理解题目果然很重要……

sarah_kim · 发表于 2011-10-5 11:13:26

谢谢！！！！也谢谢LS哈

itwm · 发表于 2012-10-3 00:59:52

第一题太坑了，怎么竟然可以这样理解啊...汗...

急问PREP数学题~求NN解答！

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