Professor Dipsy Doodle gives six problems on each exam. Each problem requires an average of 30 minutes
grading time for the entire class of 15 students. The grading time for each problem
is exponentially distributed, and the problems are independent of each other.
(a) What is the probability that the Professor will finish the grading in 2.5 hours or less?
(b) What is the most likely grading time?
(c) What is the expected grading time?