got the answer from 統計專家Commentate。 看來我是正確的。

來源: 於 2010-01-11 08:46:30 [博客] [舊帖] [給我悄悄話] 本文已被閱讀：次

explain from Commentate:
It is illustrative to show the 2nd draw's probability is a/(a+b).

Obviously, the first draw P(white) = a/(a+b), P(red) = b/(a+b).
now the 2nd draw:
P(white) = P(1st draw white)*P(2nd draw white) + P(1st draw red)*P(2nd draw white)
= a/(a+b) * (a-1)/(a+b-1) + b/(a+b) * a/(a+b-1) = a/(a+b)

so the answer is : a/(a+b)