WNAE, an all-news, all-the-time station, finds that the distribution of the length of time listeners are tuned to the station is normal, with a mean of 15 minutes and a standard deviation of 3.5 minutes. What is the probability that a particular listener will tune for:
a. More than 20 minutes?
b. For 20 minutes, or less?
c. For between 10 and 12 minutes.
I know the mean and standard deviation listed above, but I do not know how to get to the probability with the answers to the following...