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The lifetime in hours of a certain make of batteries follows a normal distribution with mean 160 and standard deviation 30.
Calculate the interval, symmetrical about the mean within which 75% of the battery lifetimes lie.

  • maths -

    The following applet takes the place of your tables or charts traditionally used for this kind of work
    http://davidmlane.com/normal.html

    click on : value from an area
    enter 0.75 in "area"
    enter 160 in "mean"
    enter 30 in "SD"

    under "results" click "between"
    click on "recalculate" to get

    125.5 to 194.5

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