Probability 救命呀 請高人指點

a. Now, 2 students are randomly chosen,

P(exactly one of the selected student passes the test)

= P(one student passes the test) X P(another student fails the test)

= (4C1 X 16C1) / 20C2

= 16/95


b.i. P(exactly two of the students pass the test)

= P(two students pass the test) X P(one student fails the test)

= (16C2 X 4C1) / 20C3

= 8/19

ii. P(at least one of the students passes the test)

= 1 - P(no student passes the test) (They are mutually exclusive)

= 1 - 4C3/20C3

= 284/285




2009-04-12 20:21:34 補充：
噢，對不起。我的式是對的，答案我寫錯了
應該是32/95

2009-04-12 20:22:09 補充：
我和你的方法其實是一樣的。
但我用了combination的方法，所以不用考慮次序。你的需要考慮次序。