A university professor who is correcting 600 written examinations decides to get a prelim- inary idea of the passing rate on the test. He decides to select a sample of exam copies as follows: She tosses an ordinary sixsided die, one for each exam copy. If the die shows a 6, she corrects the corresponding exam, otherwise not. Suppose that the selected sample consists of 90 students and that 60 out of these are found to have passed. (a) What sampling design did the professor use? (b) Give a point estimate of the proportion of students Who have passed. (0) Construct an 95% condence interval for the proportion of students who have passed