WNAE, an all-news AM station, finds that the distribution of the lengths of time listeners are tuned to the station follows the normal distribution. The mean of the distribution is 15.0 minutes and the standard deviation is 3.5 minutes. What is the probability that a particular listener will tune in for:
a. More than 20 minutes?
b. 20 minutes or less?
c. Between 10 and 12 minutes?