问PREP数学题~求NN解答！

yaozhixinjie

106.

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

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【答案】A

62.
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！！

pye348789

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

yaozhixinjie

为什么要*3呢？？

pye348789

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

julia_juer

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~~

yaozhixinjie

谢谢！！！

ashleych

嗯，我也求解第一个题目的解释，不是很明白……