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Binomial distribution

  1. Aug 25, 2012 #1
    1. The problem statement, all variables and given/known data
    I've uploaded a picture of the question.
    I need help in identifying the correct number of trials, probability of success and the X value(number of successes)

    2. Relevant equations
    i'm using the binomial distribution function on the calculator but i've attached the formula just in case


    3. The attempt at a solution
    I used n=7, p=7/15 and X=7 which yields me a probability of 0.00482 which is incorrect. The correct answer is 0.000155 but not sure how.
     

    Attached Files:

  2. jcsd
  3. Aug 25, 2012 #2

    LCKurtz

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    The binomial distribution is inappropriate for this problem. You aren't doing independent trials with replacement. Think about how many ways you can select 7 cars from the 15 and how many ways you can select the 7 fwd cars.
     
  4. Aug 25, 2012 #3

    Dick

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    That's not a correct formula to use. How many ways are there to choose 7 cars from 15? Only one of those choices gives you all four wheel drives.
     
  5. Aug 25, 2012 #4
    ahh, thats why i keep getting it wrong. Thanks, but if its not binomial, what is it?
     
  6. Aug 25, 2012 #5

    Dick

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    It's combinatorics. How many ways to select 7 objects from 15 objects?
     
  7. Aug 25, 2012 #6

    Ray Vickson

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    It is the so-called hypergeometric distribution.

    Note: instead of a combinatorial argument there is another way to get the correct answer. The probability that the first car is fwd is 7/15; that leaves 14 cars, of which 6 are fwd. So (given the first is fwd) the probability that the second is fwd is 6/14, etc, etc.

    RGV
     
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