Posted by **jake** on Saturday, March 12, 2011 at 11:39am.

If there are 6 blue marbles and 2 red marbles in a bag, what is the probability of picking a blue marble then another blue marble if you do not replace the first marble P(B,B)? 3/4 x 5/7 or is it 3/4 x 6/7?

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**Reiny**, Saturday, March 12, 2011 at 11:57am
Prob = (6/8)(5/7) , which was your first choice

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**jake**, Saturday, March 12, 2011 at 12:36pm
3/4 x 5/7, but that means you presume you took out a blue marble the first time. How do you know that you didn't take out a red marble, so you still have all six blue marbles when you pick the second time?

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**Reiny**, Saturday, March 12, 2011 at 2:26pm
"... but that means you presume you took out a blue marble the first time"

yes, that is the case we wanted, and the 6/8 is the probability that would have happened, and one of the blue marbles has now been picked.

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