According to the National Survey of Student Engagement, the average student spends about 17 hours each week preparing for classes; preparation for classes includes homework, reading and any other assignments. (Data extracted from "How much do you study...," usat.ly/1QrG1aj.) Assume the standard deviation of time spent preparing for classes is 4 hours. If you select a random sample of 16 students,

a. What is the probability that the mean time spent preparing for classes is at least 16 hours per week?

b. There is an 85% chance that the sample mean is less than how many hours per week?

c. What assumption must you make in order to solve (a) and (b)?

d. If you select a random sample of 64 students, there is an 85% chance that the sample mean is less than how many hours per week?