perplexus.info :: Probability : Billy's bag

Billy's bag (Posted on 2005-04-17)

On a table there are two bags. The first contains one white marble. The second bag contains one red and one white marble. Blindfolded, Billy takes one marble out of the second bag and puts it in the first bag. He then takes off his blindfold and takes a marble out of bag one. It is a white marble.
What is the probability that the other marble in the first bag is also a white marble?

Dutch National Science Quiz 12/24/2004

Submitted by Hugo

Rating: 3.0000 (2 votes)

Solution: (Hide)

The probability that the remaining marble is also white, is 2/3.
After adding the first marble, there are either two white marbles (W1 and W2) or one white (W1) and one red marble in bag one.
You can have two ways to pick a white marble and still have another white in the bag: you can take out W1 and have W2 still inside or the other way around.

Comments: ( You must be logged in to post comments.)

Subject	Author	Date
Puzzle Solution	K Sengupta	2008-12-14 06:55:55
Answer	K Sengupta	2008-12-12 13:04:20
No Subject	Sean	2005-10-26 13:31:38
a wrong guess?	tanx	2005-10-15 00:21:26
ddddddd	Andy	2005-05-07 19:08:47
ddd	Andy	2005-05-07 19:08:30
re(2): more manually	dqbombo	2005-05-02 22:54:33
re(2): more manually	armando	2005-04-19 15:56:16
If Billy is looking	owl	2005-04-19 03:09:55
re: more manually	Bruce	2005-04-18 21:16:48
re: 3 cases	armando	2005-04-18 16:00:46
3 cases	The riddler	2005-04-17 22:59:49
more manually	armando	2005-04-17 21:56:02
No Subject	Stephen Ticsay	2005-04-17 21:16:38
re(2): Solution	TomM	2005-04-17 17:54:24
re(2): Solution	np_rt	2005-04-17 17:17:14
re: Solution	Charlie	2005-04-17 17:04:01
Solution	np_rt	2005-04-17 16:44:44

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