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Math Help - Standard deviation and mean?

  1. #1
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    Standard deviation and mean?

    The duration of direct flights from London to Singapore in a particular year followed a normal
    distribution with mean μ and standard deviation σ .
    92 % of flights took under 13 hours, while only 12 % of flights took under 12 hours 35 minutes.
    Find μ and σ to the nearest minute.

    I'm stuck here, i put in the formula and then how do i find inverse normal? Step by step instructions? I have a TI84 if that helps.
    Thanks
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  2. #2
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    Re: Standard deviation and mean?

    Quote Originally Posted by Koko1211 View Post
    The duration of direct flights from London to Singapore in a particular year followed a normal
    distribution with mean μ and standard deviation σ .
    92 % of flights took under 13 hours, while only 12 % of flights took under 12 hours 35 minutes.
    Find μ and σ to the nearest minute.

    I'm stuck here, i put in the formula and then how do i find inverse normal? Step by step instructions? I have a TI84 if that helps.
    Thanks
    Usually you look it up in a table.

    For a standard normal

    92% corresponds to a z-score of 1.405

    12% corresponds to a z-score of -1.175

    That should be enough to finish the problem.
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