Question 1100449
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P((urn B1) AND (black)) = {{{(1/2)(3/5) = 3/10}}}<br>
P((urn B2) AND (black)) = {{{(1/2)(4/7) = 2/7}}}<br>
Then the probability that urn B1 was chosen, given that the ball is black, is {{{(3/10)/((3/10)+(2/7)) = (3/10)/(41/70) = 21/41}}}