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Homework Help: Normally Distributed.

  1. Mar 2, 2010 #1
    The life of a light bulb manufactured by a company is found to be normally distributed. if the bulbs last 400 days with a standard deviation of 75 days, what is the probability that a randomly chosen light bulb lasts?

    a) at least 450 days?
    b) less than 150 days?

    Throughout the examples in the book im studying, i am not making much sence of this.
    Would be appreciated for some advice were to start.

  2. jcsd
  3. Mar 3, 2010 #2


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    Homework Helper

    You can say that the 400 is the mean. They gave you the standard deviation, so you can find the variance.

    Then you just need to 'standardize' the distribution as tables will give you P(Z) where Z=(X-μ)/σ
  4. Mar 3, 2010 #3
    Well thank makes it a bit more clear, Thanks for that.
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