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Homework Help: Conditional Probability Statistics

  1. Jan 14, 2013 #1
    1. Assume that there is a 50% chance of hard drive damage if a power line to which a computer is connected is hit during an electrical storm. There is a 5% chance that an electrical storm will occur on any given summer day in a given area. If there is a 0.1% chance that the line will be hit during a storm, what is the probability that the line will be hit and there will be hard drive damage during the next electrical storm in this area?

    2. Bayes Theorem? P(A|B) = (P(B|A) * P(A))/ P(B)

    3. I'm thinking it has something to do with Bayes Theorem...
    P(hard drive damage) = 0.5
    P(electrical storm) = 0.05
    P(line hit) = 0.001

    I started off by figuring out what is the probability of the line getting hit:
    P(l|es) = (0.001 * 0.05)/ 0.05 = 0.001
    but I'm not sure what I should do next and if my P(l|es) is correct.
  2. jcsd
  3. Jan 14, 2013 #2


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    The circumstance is 'the next storm', and you know the probability of a hit during such a storm. The probability of a storm is irrelevant.
    So you have: prob of a hit; prob of damage given a hit.
  4. Jan 14, 2013 #3


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    Here is how I would do this problem:
    Since the problem says "during the next electrical storm", the "There is a 5% chance that an electrical storm will occur on any given summer day in a given area" is irrelevant- we are given that there is a storm. Suppose there are 1000000 electrical storms. There is a .1% (.001) chance of a line being hit by lightning. So out of 1000000, how many lines are hit by lightning? Of those, 50%= .5 will result in hard drives damaged. So of those 1000000 storms, how many drives are damaged? What proportion is that?

  5. Jan 19, 2013 #4
    Thank You!!!
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