F 5 Maths 概率

因為只能在錢包中取出兩枚硬幤
只有以下情況才能令兩枚的總值大於$2.6
1枚一元+1枚二元
2枚二元

硬幣總數=2+3+2=7

概率
= [(3C1‧2C1)+(2C2)] / (7C2)
= (6+1) / 21
= 1/3

3C1 = 從3枚一元硬幣中取出1枚一元硬幣
2C1 = 從2枚二元硬幣中取出1枚二元硬幣
2C2 = 從2枚二元硬幣中取出2枚二元硬幣
7C2 = 從7枚硬幣中取出2枚硬幣

2014-03-17 19:39:38 補充：
很好的例子~
值得參考
=]

2014-03-17 19:44:44 補充：
批卷貓提供了一些值得參考的例子

http://hk.knowledge.yahoo.com/question/question?qid=7014011400230

http://hk.knowledge.yahoo.com/question/question?qid=7014011700083

我和同事都在這家購買；http://*****/h4oid 不錯的說！！

>快去這里*****我每次都是去這里看
乞僵伭佫俧

>這里很不錯ａａａｓｈｏｐｓ。ｃｏｍ老婆很喜歡
卅僠

Probability of drawing a 1 dollar coin = 3/7.
Probability of drawing a 2 dollar coin = 2/7.
For total value to be greater than $2.60, it should be two 2 dollar coins or one 1 dollar coin and one 2 dollar coin.
P(2 dollar coin in 1st draw and 2 dollar coin in 2nd draw) = (2/7)(1/6) = 1/21.
P(2 dollar coin in 1st draw and 1 dollar coin in 2nd draw) = (2/7)(3/6) = 1/7.
P(1 dollar coin in 1st draw and 2 dollar coin in 2nd draw) = (3/7)(2/6) = 1/7.
So total probability = 1/21 + 1/7 + 1/7 = 7/21 = 1/3.

HK~

可能 Lily 不會看意見欄～

你可以補充回答貼上以下兩個連結給她參考一下～

http://hk.knowledge.yahoo.com/question/question?qid=7014011400230

http://hk.knowledge.yahoo.com/question/question?qid=7014011700083

大家努力！

2014-03-17 19:42:03 補充：
其實 drawing without replacement 的逐個分數乘，其實即係做緊 nCr 的計算，不過同學未必一定明白，所以兩個solutions都睇睇比較好～