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Posted by on Sunday, December 2, 2007 at 8:32pm.

The amount of Jen's monthly phone bill is normally distributed with a mean of $50 and a standard deviation of $12. What percentage of her phone bills are between $14 and $86?

  • Math Quantitive Reasoning - , Sunday, December 2, 2007 at 8:56pm

    the z score for $14 is (14-50)/12 = -3
    the z score for $86 = (86-50)/12 = +3

    at this point you will need a table of values of z scores, or some distribution calculator
    Here is a rather simple one

    http://davidmlane.com/hyperstat/z_table.html

    You could either enter
    mean: 0
    standard deviation: 1
    between -3 and +3

    or for this one we didn't even have to calculate the z-scores and could have entered
    Mean 50
    SD 12
    between 14 and 86, notice we got the same result, .9973 which is 99.73%

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