1. At a department store catalog orders counter, the average time that a customer has to wait before being served has been found to be approximately exponentially distributed, with a mean (and standard deviation) of 3.5 minutes. For a simple random sample of 36 recent customers, invoke the central limit theorem and determine the probability that their average waiting time was at least 4.0 minutes.
2. It has been reported that the average U.S. teenager sends 80 text messages per day. For purposes of this exercise, we will assume the daily number of text messages sent is normally distributed with a standard deviation of 15.0 messages. For a randomly selected group of 10 teenagers, and considering these persons to be a simple random sample of all U.S. teens, what is the probability that the group will send at least 900 text messages (i.e., a sample mean of at least 90 messages per teen) next Wednesday?
Source: healthnews.com, May 27, 2009.