Total inventory = 1500 , 1971 was 10 \% of total inventory thus, .01 \times 1500 = 150 . Now, the question says that “300 textbooks were purchased” and all of these textbooks were either counted in or discarded and after that they’re asking the percentage of the book discarded. Now, from chart and earlier calculation we know that 1971 had 150 books in inventory thus, 300-150 = 150 books were discarded (or half) . Hence, we can choose 50\% that is the same as saying half of the books were discarded!