Question 861011
Since you know it is not black then it is as if there are no black marbles in the box, only white and red.
{{{P(W)=5/8}}}

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)={{{5/12}}} (WWWWW intersecting with WWWWWRRR gives you WWWWW out of 12)
P(B) is probability that a marble is not black={{{8/12}}}
P(A|B)={{{(5/12)/(8/12)=5/8}}} as in our intuitive answer above.