There are 3 balls in a bag, they are black(B), red(R) and white(W) respectively. Suppose the probability of drawing B is 0.1, R is 0.4 and W is 0.5. You draw a ball and it is known that it is black or red(that means it cannot be white). What is the probability of drawing a black ball?
Originally: 0.1+0.4+0.5=1
As drawing white ball is impossible, we cancel it out and it becomes
0.1+0.4=0.5......(*)
(*) x 2 to have one as the utimate possibility 1
0.2+0.8=1
Thus the probability of drawing a black ball is 0.2.
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