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Math Help - Normal Distribution Help!!

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    Normal Distribution Help!!

    The mean lifetime of a wrist watch is 25 months, with a standard deviation of 5 months, the distribution is normal. For how many months should a guarantee be made if the manufacturer doesnt want to exchange more than 10% of the watches?

    Im having trouble in understanding how to figure out the area under the distribution for this, calculating is fine.
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  2. #2
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    Quote Originally Posted by lynchjp50 View Post
    The mean lifetime of a wrist watch is 25 months, with a standard deviation of 5 months, the distribution is normal. For how many months should a guarantee be made if the manufacturer doesnt want to exchange more than 10% of the watches?

    Im having trouble in understanding how to figure out the area under the distribution for this, calculating is fine.
    You require the value of x* such that Pr(X < x*) = 0.1.

    Use your standard normal tables in reverse to find the value of z* such that Pr(Z < z*) = 0.1.

    Then from Z = \frac{X - \mu}{\sigma} you have z^* = \frac{x^* - 25}{5}.

    Substitute the value of z* and solve for x*.
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