Applications of Normal Distribution

berrysryummy

New member
Here is the problem:

Megan drives to work each morning. Her commute time is normally distributed with mean 30 minutes and standard deviation 4 minutes. Her workday begins at 9:00AM. At what time should she leave for work so that the probability she is on time is 95%?

I know the value of x bar, sigma, and z of alpha over 2, but I have no idea where to get n. Am I using the completely wrong formula?

x bar = 30
sigma=4
z of alpha over 2= 1.95996

I'm using x bar +/- z* alpha times sigma over the square root of n.