中五數學-概率問題

P( 第二個球是紅色 )
= 2/4
= 1/2

Probability of getting a white ball in the 1st draw = P(A) = 3/5.
Probability of getting 1 white ball and then 1 red ball without replacement
= P(A and B) = 3/5 x 2/4. (Multiplication Rule)
By the rule of conditional probability, probability of B giving than A has happened = P(B/A) = P(A and B)/P(A).
So probability of drawing red ball giving that white ball has drawn
= (3/5 x 2/4)/(3/5) = 2/4 = 1/2.

由於第一次已抽出白球一個,所以分母是3(白球)+2(紅球)-1(白球),而第二個球被抽出的機會仍然是2,分子故作為2(因第一次沒被抽出),所以本題答案是2/4,約簡後即是1/2或50%.