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Homework Help: Simple probability question

  1. Jan 11, 2005 #1


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    I absolutely hate stats! its got to be one of the worst topics going.Anyway here is the (simple) question i am having (lots) of problems with! (im revising over and cant remember a thing!)

    "In each of the following questions, list a sample space consisting of equally likely outcomes and hence find the required probabilities"

    1) There are 5 counters in a bag, each marked with a different number from the set {1,2,3,4,5} .Two counters are drawn from the bag,Find the probabaility the total score on the counters is

    a) greater than 7
    b) a prime number*
    c) an odd number*

    I made up a sample space diagram and i find out that out of 20 possible outcomes, 4 added up to be greater than 7. So my answer was 1/5

    However i reread it and i done it through another way ... i said:

    p(grater than 7) is the probabaility we get... p(3,5) + p(5,3) + p(4,5) + p(5,4)

    (ok so this is essentially the same) BUT the probability of getting a 3 is 1/5 right? so we get (1/5 X 1/5) X 4 = 4/25

    so ... which one i correct?

    Please, please help!


    * i think ill attempt these again once i know the answer for this.
  2. jcsd
  3. Jan 11, 2005 #2


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    That's correct. It was unnecessary to consider (3,5) different from (5,3) though. There are [itex]{5 \choose 2} = 10[/itex] ways of picking two counters and only (3,5) and (4,5) give a sum greater than 7. So the prob. is 1/5.
    Same answer and both methods are valid.

    Remember that once you have taken a counter (say 3) out of the bag there are 4 left. So the probability of picking the 5 after you've taken the 3 is not 1/5, but 1/4.
    Last edited: Jan 11, 2005
  4. Jan 11, 2005 #3


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    thank you Galileo!

    Relieved - Mo
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