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A box  contains 10 red balls and 15 Blue Balls.One ball is lost. Two balls are drawn from box and are found to be red ones.Find the chance that the missing ball is blue ball.

a)15/23

b)0.347

c)0.6

d)none

1 Answer

Best answer
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There are 2 possibilites :

Case 1 : Lost ball is red :

In that case probability that both balls are red  =  9/24 * 8/23  =  3 / 23 [Assuming we are drawing balls without replacement]

Case 2 : Lost ball is blue :

In that case probability that both balls are red  =  10/24 * 9/23  =  15/92

So  P(lost ball is blue)  :  Case 2 / (Case 1 + Case 2)

                             =      15/92 (  12/92 + 15/92)

                             =      15/27

                             =      5/9    =   0.556(correct to 3 decimal places)

Hence D) should be the correct answer..

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