Because the upper layer is more than the middle layer 1 1, and the lower layer is less than the middle layer by 5, we can know that the lower layer is less than the upper layer 16.

So the number of books upstairs is (108+11+16)/3 = 45 (Ben).

The number of books in the middle layer is 45- 1 1=34 (books).

The number of books in the lower level is 34-5=29 (this book)

I hope this answer can help you.