Professor Snape and the Correction

Umbridge is known to conduct a lot of examinations in Hogwarts. Due to this, Professor Snape is in trouble because of so many answer sheets to be corrected. He wants to minimize the average waiting time of each student. Hence, he decides which paper he corrects first. Every answer answer sheet takes a different amount of time to be checked. Also, when he starts correcting a paper, he cannot corr