A class has 420 students. The final examination is optional-taking it can raise, but cannot lower, a

Question:

A class has 420 students. The final examination is optional-taking it can raise, but cannot lower, a student's grade. Of a random sample of 80 students, 31 indicated that they would take the final examination.
Find a 90% confidence interval for the total number of students in this class intending to take the final examination.