A bookshelf has two layers of books, and the books on the upper layer account for 40% of the total; if 48 books are taken from the upper layer and placed in the lower layer, then the books on the lower layer account for 75% of the total. How many books are there in this bookshelf?
一个书架有两层书，上层的书占总数的40%；若从上层取48本放入下层，这时下层的书占总数的75%．这个书架共有多少本书？
<result>320</result>