正态分布和百分比互求的题目怎么解答？

GmatFighter1

There are these 8 numbersin a set 9.4,9.9,9.9,9.9,10.0,10.2,10.2,10.5 The mean and standard deviation ofthe 8 numbers are 10.0 and 0.3 respectively, what percent of the 8 numbers arewithin 1 standard deviation of the mean?
a)90%
b) 85%
c) 80%
d) 75%
e) 70%


解答看懂了
但是题目其实有疑问
为什么８个数的中数是１０呢
不是因为是9.9和10的平均数么？


ｐｓ
还有一道题目
The fourth grade at school X is made up of 300 students who have a total weight of 21,600 pounds.
If the weight of these four graders has a normal distribution and standard deviation equals 12 pounds, approximately what percentage of the fourth graders weighs more than 84 pounds?

a 12%
b 16%
c 36%
d 48%
e 60%


网上看了解释也看不明白
Where did you get this question? My believe is that GMAT will not be asking questions like this....

Mean = 21600/300 = 72

Mean + 1sigma (std dev) = 72 + 12 = 84

+/- 1<sigma> covers 68.2% (34.1% + 34.1%) of the normal distribution. Therefore, remain distribution of data 100% - 68.2% = 31.8%.

Since we care only about data points above 84 lbs => 31.8 / 2 = 15.9% = 16%


怎么就一下子两个ｓｉｇｍａ就得出６８．２的百分比了阿？
ＧＭＡＴ关于正态分布到底要掌握到哪里？

sunnycc

LZ，第一题是说平均数是10不是中数吧。跟平均数相差一个方差以内的数占的百分比

sunnycc

ding,第二题求解啊

ashcroft

第一题是80%么？
因为题目告诉了mean是10，ps，你说的那个是中位数，那个应该是median~不是mean

ashcroft

34.1%是个常数。。。背下来的。。。

toprunning

第二提， 不是在9.7和10.3之见吗？这样不是75%吗？

GmatFighter1

第一题是80%么？
因为题目告诉了mean是10，ps，你说的那个是中位数，那个应该是median~不是mean

-- by 会员 ashcroft (2011/9/30 14:06:43)

答案是７５


mean = 10
sd = 0.3

"one standard deviation" = 1*(sd) = 0.3.

two standard deviations would be 2*(sd) = 0.6,etc.

A number that is within SD away from the meancould be greater or less than the mean -- in this case, it could be either ofthe following:

greater than 10 - 0.3 = 9.7
less than 10 + 0.3 = 10.3

GmatFighter1

34.1%是个常数。。。背下来的。。。

-- by 会员 ashcroft (2011/9/30 14:43:14)

背下来。。。那不是题目其他条件都没用了？
７２，８４不管什么都是３４．１？？？！！