According to a social mediablog, time spent on a certain social networking website has a mean of 21 minutes per visit. Assume that time spent on the social networking site per visit is normally distributed and that the standard deviation is 5 minutes.

a. If you select a random sample of 16 sessions, what is the probability that the sample mean is between 20.5 and 21.5 minutes?

b. If you select a random sample of 16 sessions, what is the probability that the sample mean is between 20 and 21 minutes?

c. If you select a random sample of 144 sessions, what is the probability that the sample mean is between 20.5 and 21.5 minutes?