MT Paper 2,probability 的问题

ting_006

1.Asni,Barun and Culian aremfrom came class of 40 students.Three students are to be selected to be the monitor,assistant monitor and treasurer .What is the probability that
a)Asni is selected to be the monitor,Barun the assistant monitor and Culian the treasurer?
ans:1/59280

b)Asni,Barun and Culian are selected to hold the three posts?
ans:1/9880

c)three out of six students in the first row are selected to hold the posts?
ans:1/494



2.5 apples are to given to a group of children of 6 boys and 2 girls.What is the probabilty that
a)a particular boy David will receive an apple?
ans:5/8    

b)at most one of the girls will receive an apple?
ans:9/14


麻烦各位帮忙解答。
谢谢！！！

woeifang

回复 1# ting_006


1 a) P(.......)=(1/40)(1/39)(1/38)

   b) P(.......)=(3/40)(2/39)(1/38)

   c) P(.......)=(6x5x4)/59280

2 a) P(.......)=5/8         <<straight away,no step...

   b)這個我的答案和你不一樣，如果我錯了 ，請share一下你的答案哦

      P(.....)=(2x6x5x4x3)/(8x7x6x5x4)
                =3/28

menglee90

回复 1# ting_006


   2b的答案：
http://www.wolframalpha.com/input/?i=%282choose0*6choose5%2B2choose1*6choose4%29%2F%288choose5%29

ting_006

回复 2# woeifang


    2.b 的答案：

n(at most one of the girls will receive an apple)(6C4 x 2C1)+6C5=36

n(sample space)=8C5
                             =56

P(at most one of the girls will receive an apple)=36/56
                                                                                =9/14

ting_006

非常感谢两为的帮忙！！！
Thank You!!!!!