Question 1052747
 mean of 15 minutes and a standard deviation of 10 minutes
the longest time a person can talk without incurring the extra charge?
<= 95%
invNorm(.95) = 1.645

{{{z =blue (x - mu)/blue(sigma)}}}
0r

 {{{blue(sigma)*z + mu=blue (x)}}}
10*(1.645) + 15 = x = 31.45 (for these always  round up to next whole number)
32min is the longest time a person can talk without incurring the extra charge