A bookstore sells a particular book of tax laws for Rs 100. It purchases the book for Rs 80 per copy. Since some of the tax laws change every year, the copies unsold at the end of a year become outdated and can be disposed of for Rs 30 each. According to past experience, the annual demand for this book is between 18 and 23 copies. Assuming that the order for this book can be placed only once during the year, the problem before the store's manager is to decide how many copies of the book should be purchased for the next year.