求助几道prep数学题

cx_ding · 发表于 2009-12-5 05:38:14

The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that Pat, a resident of Town X, watched television last week was between 1 nad 2 standard deviations below the mean. Which of the following could be the number of hours that Pat watched television last week?
30 20 18 12 6

An integer greater than 1 that is not prime is called composite. If the tow-digit integer n is greater than 20, is n composite?
1 The tens digit of n is a factor of the units digit of n.
2 The tens digit of n is 2.

a10285 · 发表于 2009-12-5 15:54:50

1、平均数为21，在平均数之下1-2标准差（6-12），则为9-15之间，选12

2、（1）举反例41
（2）举反例23
选E

cx_ding · 发表于 2009-12-6 07:13:39

谢谢楼上解答
第2题懂了
第1题还是不懂，prep的解释也看了，看不懂，不知道这一类题的解法是什么，我只知道标准方差的算法，但是不会用。
请指教

a10285 · 发表于 2009-12-6 12:36:25

不好意思，我2#的解释好像有些偏差，已更改，请LZ帮忙确认·~
标准差是一组数值自平均值分散开来的程度的一种测量观念。一个较大的标准差，代表大部分的数值和其平均值之间差异较大；一个较小的标准差，代表这些数值较接近平均值。
比如说平均值+2*标准差，表明在2个标准差以内的值
而本题中应该是[平均值-2*标准差, 平均值-1*标准差]
不知这样解释行不行~~

cx_ding · 发表于 2009-12-7 08:54:29

平均值+X*标准差是不是这类题目的公式？

a10285 · 发表于 2009-12-7 13:35:27

没错~

cx_ding · 发表于 2009-12-7 14:21:43

多谢多谢，这可解决我一直以来对这类问题的不解。

neversight · 发表于 2009-12-7 23:05:45

The tens digit of n is a factor of the units digit of n.
这个不是说十位是个位的因子吗？4是1的因子？
不懂呢？能不能给指点一下啊？谢谢

juange1014 · 发表于 2009-12-10 16:36:52

An integer greater than 1 that is not prime is called composite. If the tow-digit integer n is greater than 20, is n composite?
1 The tens digit of n is a factor of the units digit of n.
2 The tens digit of n is 2.

-- by 会员 cx_ding (2009/12/5 5:38:14)

[/quote]

How about answer A

let "ab" represent n, then n=10a+b

1) The tens digit of n is a factor of the units digit of n==>b=ak(k >=1, integer)==> n=10a+b=10a+ak=ak(10+1) see,n is greater than 20, then n is a composite, sufficient!

2) The tens digit of n is 2, n could be any number from 21-29, could be prime (23,29) or composite(21, 24...) insufficient...

求助几道prep数学题

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