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Homework Help: Bayes theorem and probability help

  1. Apr 13, 2009 #1
    1. The problem statement, all variables and given/known data
    'A dashboard warning light is supposed to flash red if a car’s oil pressure is
    too low. On a certain model, the probability of the light flashing when it should is 0.99; 2%
    of the time, though, it flashes for no apparent reason. If there is a 10% chance that the oil
    really is low, what is the probability that a driver needs to be concerned if the warning light
    goes on?'

    2. Relevant equations

    Law of total probability/bayes theorem?

    3. The attempt at a solution
    having trouble making a start..
  2. jcsd
  3. Apr 13, 2009 #2


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    Gold Member

    Re: Probability...

    Perhaps try drawing a tree diagram to help represent the situation.
  4. Apr 13, 2009 #3
    Re: Probability...

    i'm not sure how to do that though,
    because of the 0.99 and the 2%,
    add up to more than 1???
  5. Apr 14, 2009 #4


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    Gold Member

    Re: Probability...

    They are for different cases though, which is why it adds up to more than 100%.

    The question states that IF the oil pressure is actually too low, then it will flash 99% of the time.

    On the other hand though, if the oil pressure is not too low, then it will still flash 2% of the time.

    Can you see how those figures are allowed to add up to more than 100%? Does that help?
  6. Apr 14, 2009 #5
    Re: Probability...

    P(concerned) = (0.99 x 0.1) + (0.02 x 0.9) = 0.117????
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