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Guest Message by DevFuse

# Pick a white, then a black, then a white

6 replies to this topic

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Posted 07 May 2013 - 03:38 AM

Three bags contain 6 marbles each; all white or black: one bag has 5 white and 1 black, another has 4 white and 2 black, the third has 3 white and 3 black. One
white marble is drawn from one bag and one black marble is drawn from another (it is not known which bags). What is the probability of drawing a white marble from the remaining bag?

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### #2 bonanova

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Posted 07 May 2013 - 06:24 AM

To solve, we need to know if the first two picks were done [a] blindly or [b] Monte Hall entered the room, looked into a bag unknown to us and removed a white marble, then went to another bag, unknown to us, looked inside and grabbed a black marble and then handed us the third bag and asks us now to guess its contents (calculate p of drawing white).

I'm thinking we should assume it's [a] and go talk with *gulp* Professor Bayes.

In that case an acceptable paraphrase of OP is: There are three bags (as described) someone grabbed a bag and with eyes closed pulled out a marble. It turned out to be white. Then he grabbed one of the other bags and pulled out, eyes closed, another marble. It turned out to be black. Setting those bags aside, we are handed the third bag and asked to say the probability of drawing white.

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Vidi vici veni.

### #3 bonanova

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Posted 07 May 2013 - 07:41 AM

Spoiler for Looks like

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Vidi vici veni.

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Posted 07 May 2013 - 04:10 PM

Spoiler for Looks like

The bags are picked at random but there is a flaw in your logic.

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### #5 superprismatic

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Not just Prismatic

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Posted 07 May 2013 - 05:02 PM   Best Answer

Spoiler for I get

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Posted 07 May 2013 - 05:22 PM

Spoiler for I get

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### #7 superprismatic

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Not just Prismatic

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Posted 07 May 2013 - 05:31 PM

Spoiler for my solution

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