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: Find the probability distribution

  1. Aug 5, 2008 #1
    1. The problem statement, all variables and given/known data

    A box of 10 flashbublbs contains 3 defective bublbs. A random sample of 2 is selcted and tested. Let X be the radom variable associated with the number of defective bulbs in the sample.

    A) Find the probability distribution of X.

    B) Find the expected number of defective bulbs in a sample.

    I did this problem, I got 7/15 probability of there being no defective light bulbs, 1/15 of there being 2, but I got 7/30 of there being 1. The 7/30 is wrong but I don't know where I'm going wrong, please give me some help. Thanks in advance.

    2. Relevant equations

    3. The attempt at a solution

    7/10*6/9 = 7/15
    7/10*3/9 = 7/30
    3/10*7/9 = 1/15
  2. jcsd
  3. Aug 5, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi Sucks@Physics ! :smile:

    (in the last line, you wrote a 7 instead of a 2 :wink:)

    As you've probably noticed, the three probabilities should add up to 1.

    Your 7/30 is the probability of the first one being defective, and the second one good … but you also need the probability of the first one being good, and the second one defective … that doubles it! :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook