zkcy401

Among 40students 28 are boy.It is found that 15boys and 5girls wear glasses.If 2students are randomly selected and it is found that these two students are girls,find the probability that only one wear glasses.？ans係7／156

There are 10 orange and 8 apples in a box.If 3 of them are randomly chosen from the box one after another without replacement,find the probability that the first one is apple given that the third one is orange ans係8／17
想問下點用nPr同nCr去諗?
唔該各位

[ 本帖最後由 zkcy401 於 2011-7-7 07:19 PM 編輯 ]

cws~

Do you mean 15 students wear glasses?

zkcy401

引用:

原帖由 cws~ 於 2011-7-7 06:57 PM 發表 Do you mean 15 students wear glasses?

Sorry我打錯左題目，多謝你提醒

夏娜醬大好

唔怪得我唔識計啦

zkcy401

引用:

原帖由 夏娜醬大好 於 2011-7-7 07:10 PM 發表 唔怪得我唔識計啦

對唔住大佬

cws~

(5C1*7C1)/40C2=7/156

夏娜醬大好

條題目有問題
題目given果2個人係女仔
所以要揾P(only 1 wear glasses | both are girls)  

7/156係P(only 1 wear glasses and both are girls)

zkcy401

引用:

原帖由 夏娜醬大好 於 2011-7-7 07:21 PM 發表 條題目有問題題目given果2個人係女仔所以要揾(only 1 wear glasses | both are girls)  7/156係P(only 1 wear glasses and both are girls)

姐係條問題有問題？我改左啦喎

夏娜醬大好

2.
??O <--- third one is orange
orange得番9個
apple8個
第一個係A : 8/(8+9) = 8/17

cws~

2. 8C1/17C1 如上解釋

zkcy401

唔該哂各位先 點解你地可以諗得咁easy既=_=

我第二題係咁諗既：
8p1*10p2/18p3 + 8p2*10p1/18p3 = (5/9) p

請問用p有冇錯？

[ 本帖最後由 zkcy401 於 2011-7-7 08:13 PM 編輯 ]

夏娜醬大好

Instead of 8p1*10p2/18p3 + 8p2*10p1/18p3, you may use:
(8)(16)(10)/18P3

Consider A ? O ,
We have 8 apples and 10 oranges, so it should be 8*something*10
For the " ? " , as 2 fruits have been chosen, number of choice is 18-2 = 16
So, it is (8)(16)(10.
Then, divide it by 18P3

zkcy401

引用:

原帖由 夏娜醬大好 於 2011-7-7 08:24 PM 發表 Instead of 8p1*10p2/18p3 + 8p2*10p1/18p3, you may use:[url=tel8)(16)(10)/18](8)(16)(10)/18[/url]P3Consider A ? O ,We have 8 apples and 10 oranges, so it should be 8*something*10For the " ? " , as 2 fruits have been cho ...

真係唔該哂你

打倒孫肥羊

第2題唔應該係8/16 X 7/17 X 10/16 + 8/18 X 10/17 X 9/16咩???
                            (app,app,or)                   (app,or,or)

Cider321

For the 2nd question, the solution should be: (8 X 16P16) / 17P17 = 8/17

The position of the orange does not matter.


If you think in the probability way, the step is even easier.

It is simply 8/17 because there are 8 apples and 9 oranges that can be freely ordered.