Questions refer to the following graph.
1977～1978 TEXTB()OK INVENTORY FOR SCHOOLS X AND Y BY YEAR OF PURCHASE (as a percent of the 1977～1978 inventory)
Note：All books were purchased new on July 1 of each year.

If School X purchased 300 textbooks in 1971 and all of these textbooks either were counted in the inventory or had been discarded before the inventory, what percent of these textbooks had been discarded