Probability Question[2]

If Roy leves Alice a message saying that they will see each other at the bus-stop tomorrow, the probabilty that they will meet at the bus-stop tomorrow is 3 / 4 ; otherwise, the probailty that they will not meet at the bus-stop tomorrow is 9 / 10 .If the probability for Roy to leave Alice such a message today is 2 / 3, find the probability that they will meet at the bus-stop tomorrow.
Using total probability
P
=(2/3)(3/4)+(1/3)(1/10)
=1/2+1/30
=8/15
The answer is C
Q.2
If two letters are selceted from the word 'FLOOR' randomly, hwat is the probabilty that they can form the word 'OR'
The probability
=2/(5P2)
=2/20
=1/10
The answer is D




2008-02-15 11:59:05 補充：
下面個位第二題做錯了﹐因為第一個不能抽r的

Q.1
p(Roy to leave Alice such a message x they will meet at the bus-stop tomorrow )+
p(Roy not to leave Alice such a message x they will meet at the bus-stop tomorrow )
=2/3x3/4+1/3x1/10=1/2+1/30
=8/15
so c

Q.2 p(the frist word is O x the second word is R)+p(the frist word is R x the second word is O)
=2/5x1/4+1/5x2/4
=1/5
so b
有錯請你話我知..thx
我會聽日再睇睇..如有其他數問題我會遲d再答..