Aptitude Discussion

Q. |
There are three similar boxes, containing (i). 6 black and 4 white balls 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**

**Suraj Chavan**

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**Pikachu**

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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)

Wrong solution.

We have to use Bayes' theorem here.

Calculate..