more about probability

(a)
The required probability
= 2/6 <-- 6 pens in bag B because no matter what colour of the pen drawn from bag A,
the new number of pens in bag will be 6.
= 1/3

(b)
The required probability
= (3/5)(3/6) + (2/5)(2/6)
= 3/10 + 3/15
= 15/30
= 1/2

Explanation:
(3/5) = probability that a red pen is drawn from bag A,
(3/6) = probability that a red pen is drawn from bag B as the new number of red pens in bag B is 3,
(2/5) = probability that a blue pen is drawn from bag A,
(2/6) = probability that a red pen is drawn from bag B as the new number of red pens in bag B is 2.

有錯請指正!

2007-10-07 00:42:37 補充：
(b)The required probability= (3/5)(3/6) + (2/5)(2/6)= 3/10 + 2/15 <--- 之前打錯了. 對不起...= 13/30 <--- 這個答案才對!

(a) a black pen?
因為黑筆全在b袋,所以不用考慮a袋的問題,但由於放了一支筆進b袋,所以b袋現應有6支筆。

2/6 x 100% = 33.33%


(b) a red pen?
將"由a袋抽出紅筆"的機率和"由b袋抽到紅筆"的機會相乘就係。
因為如果a袋抽出紅筆的話,b袋會有3支紅筆和共有6支筆,
所以：

( 3/5 x 3/6 )x100 %=30%