Recent research suggests that Americans make an average of 10 phone calls per day (CNN , August 26, 2010). Let the number of calls be normally distributed with a standard deviation of 3 calls.
a. What is the probability that an average American makes between 4 and 12 calls per day?
b. What is the probability that an average American makes more than 6 calls per day?
c. What is the probability that an average American makes more than 16 calls per day?