SOLUTION: Please help. I thought I understood this but the answer was incorrect. Please show me how you got the correct answer. Thank you in advance A box contains 5 white, 3 red, and 4 bl

Question 861011: Please help.
I thought I understood this but the answer was incorrect. Please show me how you got the correct answer. Thank you in advance
A box contains 5 white, 3 red, and 4 black marbles. One marble is chosen at random, and it is not black. Find the probability that it is white. (Enter your answer as a fraction.)
Answer by Fombitz(32388) (Show Source): You can put this solution on YOUR website!
Since you know it is not black then it is as if there are no black marbles in the box, only white and red.

Using conditional probability,
Event A is the marble is white.
Event B is the marble is not black.
P(A|B) is the probability that the marble is white knowing it's not black.
P(A int B) is the intersection of the marble is white and the marble is not black.
P(B) is the probability that the marble is black.
P(A|B)=P(A int B)/P(B)
P(A int B)= (WWWWW intersecting with WWWWWRRR gives you WWWWW out of 12)
P(B) is probability that a marble is not black=
P(A|B)= as in our intuitive answer above.
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