a math JJ to discuss

onroad

有十個球，一個綠，一個藍，依次取三個球不放回，求第一個是綠，其他二個不是藍的概率
-答案： 7/30

I got the answer: 1/10 *C8,2/C9,2=7/90

???

isenetlab12

C8,2/c10,3=7/30
why C8,2? I see you mean the last two balls are chosen among the 8 other balls,but the first ball may be the blue one or one of the 8 balls since your formula did n't exclude these situations.
I think the answer should be 7/90.

onroad

Hi, thank you for your answer.
But the green and blue ball were not seprated from the other 8 balls.
And the propability of green ball is not 100% .
Would you explain it in detail ?

leqi

C8,2/c10,3=7/30