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# A BAG CONTAINS 4 BALLS. TWO BALLS ARE DRAWN AT RANDOM, AND ARE FOUND TO BE WHITE. WHAT IS THE PROBABILITY THAT ALL BALLS ARE WHITE?

Asked by 6th February 2011, 12:00 AM
Dear Student,
Here is the solution:

We need to compute P(all balls white | drew 2 white balls).
This is a conditional probability so we use Bayes Law.
P(A | B) = P(B | A) * P(A)/ P(B).

Where A = all balls white
B = draw 2 white balls

P(B) is not given. I will assume there are 3 possible worlds: 4 whites, 3 white and 1 non white,
and 2 whites and 2 non-white. I need to also know the probabilities for each of these worlds. I will assume they are equal, the standard assumption if no information is given.

P(B) = P(B| 4W)*P(4W) +P(B| 3W, 1non-white)*P(3W, 1 non-white) + P(B | 2white,2 non-white).

P(B|4W) = 1
P(4W) = 1/3 = P(3W and 2 non) = P(2W and 2 non)

P(B | 3W and 1 non) = 3/4*2/3 = 1/2

P(B | 2W and 1 non) = 2/4*1/3 = 1/6

Putting this all together with Bayes:
P(4W in bag | drew 2 white) = 1* 1/3/ ( 1*1/3 + 1/2*1/3 + 1/6*1/3) = 3/5
Regards
Team Topperlearning
Answered by Expert 7th February 2011, 4:58 PM
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