There are 1500 students at a college. From previous research, we know that the standard deviation of the number of hours that these students study outside the class per week is 10 hours. We randomly select 50 of these students and ask how many hours they study outside the class per week. It turns out that the average of these study hours is 30 hours. We want to construct a 95% confidence interval for the average number of hours that students study outside the class per week. What is our upper critical value