Question 99847
A bag contains red & blue marbles. you pick 2 marbles at random without replacing the first one. The probability of drawing a blue and then a red is 4/15. The probability that your second marble is red if your first is blue is 2/3. Find the probability the first marble is blue. 
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P(red and blue) = P(red | blue}*P(b)
4/15 = (2/3)*P(b)
P(blue) = (4/15)*(3/2) = 2/5
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Cheers,
Stan H.