1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Probability: Defective Parts

  1. May 19, 2014 #1
    1. The problem statement, all variables and given/known data

    A manager must assess the quality of a new batch of 25 components ready for shipping.
    Rather than assess each component, a sample of 5 is randomly selected and tested. The
    quality control speci fication is that if there are 2 or more defectives in the sample, the
    quality manager must reject the batch. Suppose there are actually 3 defective components
    in this batch of 25.

    What is the probability that the batch is rejected?

    2. Relevant equations

    3. The attempt at a solution

    So I'm looking for the probability that 2 or 3 defective parts are taken in the sample of 5. I'm having trouble wrapping my head around the problem a bit.

    I said the probability of choosing all 3 is:


    What's confusing me is that this has only three picks when there's five parts picked in the sample. Am I missing something here? Any help would be much appreciated.
  2. jcsd
  3. May 19, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You have 3 bad and 22 good in the batch. If x is the number of bad, how many ways can you pick x out of the bad and 5-x out of the good? Have you studied the hypergeometric distribution yet?
    Last edited: May 19, 2014
  4. May 20, 2014 #3
    This is a sample exam question yes so I would've done hypergeometric distribution yes. I just don't quite understand how to use it so I never thought that's what this problem was.

    I think I know now when it should be used though. So:



    where N=25, m=3, k=2, n=5 and use the same approach for P(X=3) except k=3.

    Is this how the problem is done?
  5. May 20, 2014 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's the idea.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted