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Geometric Distribution?

  1. May 6, 2009 #1
    Geometric Distribution???

    Geometric Distribution: In an experiment, a die is rolled repeatedly until all six faces have finally shown.?
    What is the probability that it only takes six rolls for this event to occur? ANSWER = 0.0007716

    What is the expected waiting time for this event to occur? ANSWER = 35 rolls

    Thanks a lot for any help.
     
  2. jcsd
  3. May 6, 2009 #2

    tiny-tim

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    Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
     
  4. May 6, 2009 #3
    Re: Geometric Distribution???

    yr sure ...i know that geometric distribution has the probability of (p)*(1-p)^(x-1) where the first success happens in x trials .

    then i thought that ,
    number to come in first trial is =(1/6)*(5/6)^(1-1)=1/6
    number coming in the second trial =(1/6)*(5/6)^(2-1)=1/6*(5/6)
    like wise

    probability will be =sigma(x goes 1 to 6 )[(1/6)(5/6)^(x-1)]....but seems that i got the argument wrong.and can u please help me on this and if u can guide me for a site where some easy examples on this it will be really helpful.
     
  5. May 6, 2009 #4

    tiny-tim

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    That formula is really for where you're interested in a fixed result each time (for example, how many 4s are there?) …

    Hint: here, you need the 2nd to be different from the 1st, the 3rd to be different from both of them, and so on :wink:

    (btw, how did you get on with that other thread?)
     
  6. May 6, 2009 #5
    Re: Geometric Distribution???

    yr then it should be 1*(5/6)*(4/6)*(3/6)*(2/6)*(1*6) is that so each time u have to remove one number ..........so it should be like this no .....but im not getting the answer., may be the answer is wrong ?or is it right .did u check it ??????anyway thanks for ur helping hands


    (what is other thread?i didnt get it ....!)
     
  7. May 6, 2009 #6

    tiny-tim

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    Yes, that's what I get.

    The answer given, 0.0007716, is 1/64 … I don't know how they get that. :frown:

    (i meant https://www.physicsforums.com/showthread.php?t=311671)
     
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