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Tricky Probability Interview Puzzle


Tricky Probability Interview Puzzle - 21 August

I have two coins.
* One of the coin is a faulty coin having tail on both side of it.
* The other coin is a perfect coin (heads on side and tail on other).

I blind fold myself and pick a coin and put the coin on table. The face of coin towards the sky is tail.

What is the probability that other side is also tail ?

For Solution : Click Here

16 comments:

  1. Replies
    1. Hi... Could you explain how u arrived at this ans. I have checked Admins ans. But it was not clear..

      Delete
    2. Let there be a good coin (1 heads 1 tails) and a bad coin (2 tails).

      The chances of you choosing each coin is 1/2. Since the probability of choosing the good coin is 1/2, given that tails is facing up, the probability of heads facing down would be 1/2*1/2 = 1/4.

      However, the probability of choosing a bad coin is 1/2, which means that the probability of tails facing downwards is 1/2. This is because no matter which tails is facing up, the other tails is facing down too.

      1-1/2-1/4=1/4. This 1/4 represents tails facing down and heads facing up. But this probability can be ignored in the context of the question because we know that tails and not heads is facing up.

      Given that tails is facing upwards, the probability of tails facing downwards would be (1/2) / (1/2+1/4) = 2/3

      Delete
    3. Total bullshit, you failed maths I can tell:
      2 possible scenario are :
      case number side shown other side
      1 A1 (H) A2 (H)
      2 A2 (H) A1 (H)
      3 B1 (H) B2 (T)
      4 B2 (T) B1 (H)

      case 4 is not possible so ans is 2/3


      Case 4 IS the case and thus is indeed possible. Your cases 1, 2 and 3 are NOT possible given the parameters of the question and the constraints of the scenario.

      There are only two possibilities for the remaining coin: that it is heads or that is has tails. Therefore there is a 1 in 2 chance that the other side is Tails. In other words, a 50% probability since probabilities are expressed as percentages.

      Gee people, I am glad you are not working on anything important in Life, or maybe you are and that is why the planet is in trouble. Some of the people making these questions and answers up are absolute idiots.

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    4. Retarded as fuck ...

      There are 2 scenarios. Either is the faulty coin or not.

      Result 1/2. Noone is asking you for all the possibilities.
      Its said that the coin is allready on his tail up.

      Im a little bit worried that you are doing something important in life...

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  2. 1/2
    if the coin is correct one then the other side is heads
    else the other side is tails too.
    thus the answer

    ReplyDelete
  3. 2/3:

    With no information about the coin, we know it will either be the "perfect" coin heads-up, the "perfect" coin tail-up, the "faulty" coin with the first tails-side up, or the "faulty" coin with the second tails-side up, with equal probability.

    We are given that we see a coin tails-side up, which reduces our possibilities to three of those equally-likely initial four choices, two of which implicate the "faulty" coin and one of which implicates the "perfect" coin.

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  4. if u r blindfold then how do u know that the face towards the sky is tail..?

    ReplyDelete
  5. HAHAHA....THE OTHER SIDE IS HEAD,,,BECAUSE I KNEW WHICH COIN HAD THE HEAD OTHER SIDE BEFORE BLIND FOLDING MYSELF AND PICKING UP THE SAME COIN...

    ReplyDelete