Probability
Aptitude

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Q.

There are three similar boxes, containing

(i). 6 black and 4 white balls
(ii). 3 black and 7 white balls
(iii). 5 black and 5 white balls, respectively.

If you choose one of the three boxes at random and from that particular box picks up a ball at random, and find that to be black, what is the probability that the ball picked up from the second box?

 A.

14/30

 B.

3/14

 C.

7/30

 D.

7/14

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Solution:
Option(B) is correct

Total number of black balls $= 6+3+5=14$

Total number of ways of picking black ball $=14$

Out of those, 3 are from $2^{nd}$ box

Required probability = 3/14


(2) Comment(s)


Suraj Chavan
 ()

Wrong solution.

We have to use Bayes' theorem here.

Calculate..



Pikachu
 ()

The probability of selecting the 2nd box is 1/3...and probability of selecting a black ball from the second box is 3/10..

selecting the box and then a black ball are 2 independent events...so u can't add up the number of balls from the individual boxes

ans=(3/10)*(1/3)