Imagine a college where say 2000 students are taking an intro stat course. Imagine from historical data, that student scores of a final exam can be viewed as approximately normal with a mean of 70 and a sd of 10. Suppose an A grade is considered to be 90 or higher. Now imagine a SRS of 100 students is selected from the group of 2000 students, where we then compute the sample mean of these 100 students.