Let us assume that at a certain university the actual time that a graduate student spends working toward his Ph.D. degree is approximately normally distributed with a mean of 200 weeks and a standard deviation of 50 weeks.

Given 1000 students beginning graduate work toward the Ph.D. at the same time, what number should still be working toward the degree after 300 weeks?

Find the number of weeks within which the middle 50 percent of students should complete their degrees. (Here we will simply ignore drop-outs and failures.)