数学提问

shake7570

2If 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

For a certain examination, a score of 58 was 2 standard deviations below the mean, and a score of 98 was 3 standard deviations above the mean.
What was the mean score for the examination?

(A) 74
(B) 76
(C) 78
(D) 80
(E) 82

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

In the arithmetic sequence t1, t2, t3, . . . , tn, . . . , t1 = 23 and tn = tn - 1 - 3 for each n > 1.
What is the value of n when tn = -4 ?

(A) - 1
(B) 7
(C) 10
(D) 14
(E) 20

At a dinner party, 5 people are to be seated around a circular table.
Two seating arrangements are considered different only when the positions of the people are different relative to each other.
What is the total number of different possible seating arrangements for the group?

(A) 5  (B) 10  (C) 24  (D) 32  (E) 120


本人的数学基础不是太好，这些题可能就是有的地方没读明白，希望有人告诉我怎么做 谢谢！！！

lincken

1. 答案是 大于20+2.5*3 或者小于20-2.5*3  选A

2. 58+2X=98-2X  推出X=8   则mean=58+16=74   选A

3. 这个是递减数列列式子就好了  tn=23-(n-1)*3=-4  可知n=10

4. 圆桌或者圆圈的排列你记住就是 (n-1)!  或者  A(5 5)/5  也就是选24

shake7570

非常感谢！！！

shake7570

不过 关于标准差的那个  为什么58+2X=98-2X  ？

还有20+2.5*3 ？ 为什么两个标准差相乘？