The University of Dallas bookstore stocks textbooks in preparation for sales each semester. It normally relies on departmental forecasts and preregistration records to determine how many copies of a text are needed. Preregistration shows 90 operations management students enrolled, but bookstore manager Curtis Ketterman has second thoughts, based on his intuition and some historical evidence, Curtis believes that the distribution of sales may range from 70 to 90 units, according to the following probability model:

Demand 70 75 80 85 90
Probability 0.15 0.30 0.30 0.20 0.05

This textbook costs the bookstore $82 and sells for $112. Any unsold copies can be returned to the publisher less a restocking fee and shipping for a net refund of $36. How many copies should the bookstore stock to achieve highest expected value?