# Difficult Probability Solved QuestionAptitude Discussion

 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

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)