GWD 10-37

[pinglee1984]

A jar contains 16 marbles,of which 4 are red,3 are blue,and hte rest are yellow.If 2 marbles are selected at random from the jar,one at a time without being replaced,what is the probability that the first marble selected will be red and the second marble selected will be blue?


A. 3/64
B. 1/20
C. 1/16
D. 1/12
E. 1/8


Ans: B

不好意思連發了兩題，看來我對這方面的題目都不大會做

我這題是用C(4,1)*C(3,1)/C(16,1)
所以一直算出來的是1/10
希望大家可以幫幫我的忙，謝謝囉......

[Isabelle]

你的分母取錯了喔....第二次的C(15,1)也要算進去！
C(4,1)/C(16,1) * C(3,1)/C(15,1) = 1/20

[pinglee1984]

痾....對喔....真是大白吃.....感謝感謝....希望考試別再粗心了.....

[Ruru]

請問依下為何分母不能採用C(16,2)阿??這跟C(16,1)xC(15,1)有什麼不同嗎

[meredo]

the firstmarble selected will be red and the second marble 表示一次取一顆

[FannyHsieh]

這題有更快的方法......不曉得有沒有錯:Q
直接以機率問題算的話：

(4/16)*(3/15)=1/20

(一開始16顆有四顆紅色)*(不放進去，所以剩下15顆有3顆藍色)

[chris8888]

沒錯, 我就是這樣算的

第一次取得red marble機率是 4/16 * 第二次取得blue marble機率是 3/15 = 1/4 * 1/5 => 1/20