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Reg: Probability

Frequent Contributor
Posts: 140

Reg: Probability

Actually i am having a Query for probability asked in a Interview


  If i have tossed a coin for 3 times what is the chance of getting Head 2 Times

they told 2 times but how.Can any one explain.


I am having 9 balls out of which i am having one ball with less weight each ball weight is 1 kg and the defect ball has 1 gm les in weight.
  I am having a balance at once 2 balls can we weighed in two pans.

What is the probaility to find the defect ball

The answer is 3 But how.

Respected Advisor
Posts: 3,124

Re: Reg: Probability

I am not sure if this is the right place for you to ask this kind of question, unless you were asking how to calculate it using SAS, which in that case, I can't provide the answer. That being said, you may get better answers(academic wise) elsewhere.


    When you say 'chance', I would reflect 'probability', which will never larger than 1 (I hope I am not too statistical anal here). Anyway, to anwer your question:

If you throw 3 times, then the total number of possible senario will be: 2(head,tail)*2*2=8

And the total number of 2-heads secario: 3*2/2=3, sorry don't know how to post the formula symbol.

Therefore the chance of your getting 2-heads is 3/8.


Can't believe they ask that in an interview. Anyway, the maxium time you need to weigh is 3. here is how:

Rondomly you picked 4 balls, one pair on each side of balance:

pair A vs. Pair B,

1. if they don't balance, then choose the lighter pair do another weighing, then you get your answer. This is the better scenario, which only takes 2 times.

2. if they do balance, then put them aside, choose another 2 pairs from remaining 5 balls, and weight

    2.1  if they do balance, then you will know the unweighed left one is the culprit. 2 times.

    2.2  if they don't balance, then it will take you another weighing to decide. This is the worst scenario, and it takes 3 times.



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