几道数学题,希望NN给个讲解,一定要说一下解题思路,谢谢

亡灵一族

The following is the probability distribution of the number of phone calls received by an office between 8AM and 9AM on a day.

X	1	2	3	4	5
P(x)	0.1	0.2	0.2	0.4	0.1

<1>the probability of at least 3 phone calls is

A:0.5    B: 0.2    C:0.3   D:0.7   E: none of these

<2> The mean number of phone calls is

A:3.2  B: 3.5  C: 11.6   D:1.36  E:1

<3> The standard deviation of phone calls is

A:3.2  B: 1.17  C: 11.6   D:1.36  E:none of these

不知道number of phone calls 是哪个，小弟刚开始学还请帮忙．

junyue

number of phone calls 是X

亡灵一族

还请高手告诉怎么解

myamanda

x代表次数,就是x=1,2,3,4,5分别的概率是p(x)那一行

亡灵一族

这几个题该怎么解，高手说说吧