有十个球，一个绿，一个蓝

renali

1.，有十个球，一个绿，一个蓝依次取三个球不放回，求第一个是绿，其它二个不是蓝的概率
-答案：确认： 7/30
思路： /

Who can tell me how to get the ans.7/30?

lisaislisa

C2,8/C3,10

^^

renali

Lisaislisa:

Can you explain to me the meaning of C2,8?

xiaoyayah

I think that it should be 7/60

xiaoyayah

sorry,there is more reasonable explaination8*7)/(10*9*8)=7/90
attention:take out one by one

renali

Xiaoyayah

I agree with you. the ans.should be 7/90

lisaislisa

C2,8 means after taken one green ball ,
the different other alternatives that may occur as I choose the other two balls
since I can not get the blue one
it should be 8...
get what I mean ?

^^
Merry X'mas to all !

renali

I finally got it. Thanks

Merry Christmas and Happy new year to you all

xiaoyayah

take out the green one:the probility is 1/10, so the ans. must be less than 1/10.

renali

xiaoyayah:
That's what I think: take out the green one:the probility is 1/10. then cannot take the blue one possibility is 1-1/9, same to 1-1/8
=> 1/10*(1-1/9)*(1-1/8)=7/90