Question

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?

Answer 1

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%