请教大家几道PREP上我怎么也看不懂的题目

两道关于standard deviation的,我实在是不明白这个standard deviation below the mean是什么意思?

1.The distribution of results of survay had a mean of 21hours and a standard deviation of 6. The number of hours of Pat(是一個人)watch tv last week was between 1 and 2 standard deviations below the mean. Pat上周可能看了多少小時的電視？
   

a.30 b.20 c.18 d.12 e.6

答案：d

2.  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 running times are more than 1 standard deviation below the mean of the 10 running times?

A.      One  B.two  C.three  D.four  E.five

ANSWER:B

3. DS If 500 is the multiple of 100 that is closest to X and 400 is the multiple of 100 that is closest to Y, which multiple of 100 is closest to X+Y?

 1)x<500

2) y<400

ANSWER E

4.The function f is defined for all positive integers n by the following rule: f(n) is the number of positive integers each of which is less than n and has no positive factor in common with n other than 1. If p is any prime number, then f(p)=?

A.p-1  B.p-2 C.(p+1)/2  D.(p-1)/2  E.2

Answer:A

后天就考试了,题目还看不懂..晕死了~

拜托大家了!

The distribution of results of survay had a mean of 21hours and a standard deviation of 6. The number of hours of Pat(是一個人)watch tv last week was between 1 and 2 standard deviations below the mean. Pat上周可能看了多少小時的電視？
                

standard deviations is 6,mean is 21, 2 standard deviations below the mean is 21-2*6=9

啊谢谢,我现在算是明白 standard deviation的意思了...

4.The function f is defined for all positive integers n by the following rule: f(n) is the number of positive integers each of which is less than n and has no positive factor in common with n other than 1. If p is any prime number, then f(p)=?

对于函数f,被定义为：对于所有正整数n，f（n）是小于n的正整数的个数，这些正整数必须和n没有共同的正因子（1除外），如果p是质数，f（p)=?

质数只有2个因子，1和本身，比p小的正整数一共P-1个，这些数字不可能和p有公因子。

3. DS If 500 is the multiple of 100 that is closest to X and 400 is the multiple of 100 that is closest to Y, which multiple of 100 is closest to X+Y?

 1)x<500

2) y<400

500是100的倍数中最靠近x的，450<X<550

400......................................Y...,   350<Y<450

800<x+y<1000   将2个条件代入800<X+Y<900，由于不知道和850的关系，所以无法确定最接近的是800还是900

I am not a NN, but try to help...

2.  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 running times are more than 1 standard deviation below the mean of the 10 running times?

A.      One  B.two  C.three  D.four  E.five

1. the mean of the 10 running times:

(70+75+80+85+90+105+105+130+130+130)/10=100

2. 1 standard deviation below:

 below 100-22.5=77.5

3. 70, 75 are the times meets the requirement --> answer B