We Would Like to Estimate the Average Amount of Time $\mathbf{9 0} \%$

Short Answer

We would like to estimate the average amount of time that all students at a certain college spent studying during Spring Weekend. We would like to be $\mathbf{9 0} \%$ confident that our estimate is within .5 hours of the actual population average.
-Find the minimum number of students that need to be sampled if it is known that the maximum and minimum amounts of hours were done by students who studied for 15 hours and 0 hours respectively.