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Homework Help: Exponential Distribution

  1. Sep 23, 2004 #1
    I am having alot of trouble with a homework question from my book. It asks:

    The time between unplanned shutdowns of a power plant has an exponential distribution with a mean of 20 days. Find the probability that the time between two unplanned shutdowns is more than 21 days.

    I know this much so far 1-e-(20)(?) (one minus e to the negative power of mean times any value of the continuous variable(X))

    I am lost on finding X within the equation.

    Hope someone can help.
     
  2. jcsd
  3. Sep 23, 2004 #2
    Hello!

    First of all what do we define as an exponential distribution? It is equaled to:

    1 - e^(-yx)

    = 1 - e^(-20)(21)

    Since this is the probability that the time between two unplanned shutdowns is less than 21 we don't need the 1. Hence our answer (I think) would be:

    e^(-20)(21) or essentially 0.

    Hope this helps!
     
    Last edited: Sep 23, 2004
  4. Sep 24, 2004 #3
    Thanks for your help,

    But how do I solve E^-(20)(21)? I know that 20 times 21 is 420. How do I determine e^-420?

    The book has given me some answers, but none say zero. a. .350, b. .650, c. .150, d. .850

    I can probabily figure out the answer with no problem if someone can help me witht the problem above.
    Thanks for your help!!!
     
  5. Sep 24, 2004 #4
    Okay, here is what I have, please tell me if I am right:

    F(x)=1-e^-(20)(2/21)

    F(x)=1-.850

    F(x)=.150 (final answer)

    I hope I am write.
     
  6. Sep 24, 2004 #5
    I meant right**** Sorry :blushing:
     
  7. Sep 24, 2004 #6
    Asking for Review

    I feel confident about my answer, I am hoping someone can review and let me know if I have calculated wrong in any way. :approve:
     
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