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 (1450)/12 = 3
the z score for $86 = (8650)/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 zscores and could have entered
Mean 50
SD 12
between 14 and 86, notice we got the same result, .9973 which is 99.73%