I need help!!!

I have a question that I am having problems with.
1. Suppose you record how long it takes you to get to work over many months and discover that the times are approximately normal with a mean of 15 minutes and a standard deviation of 2 minutes. How much time should you allow to get there to make sure you are on time 90% of the time? What I have done so far. I have set up model with the times 7, 9, 11, 15, 17, 19, 21 and made a normal distribution curve. I figured that about 90% would lie around 18. I know that I need to do something with the z-score table but I am a little cofused about what.
Find the z-score where 90% (0.9) of the area under the normal curve is to the left of it. You can look this up in a table.

Then using the familiar formula for finding z-score from mean, standard deviation, and observed value, set z to the value you looked up in the table and solve the rest for the unknown value (you already know the mean and std). This is the time you are looking for.

See if the answer is around 18 minutes.