Posted by **Lindsey** 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 -
**Reiny**, 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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