Purdue Book Store (the store) needs to decide how many copies of a textbook to purchase from McGraw Hill (the publisher) before the start of the next semester. The retail price is $28 per copy at the store. The publisher produces at the cost of $15 per copy and sells to the store at a wholesale price of $18 per copy. The store will dispose of all unsold copies at 50% off the retail price at the end of the semester. It is estimated that demand for this book during the semester is Normal with mean 1000 and standard deviation 300. The store's optimal order quantity is