Suppose there is a book on the first floor, a book on the second floor and a book on the third floor.

After taking the book: the first floor: a-b, the second floor: 2b-c, and the third floor: 2c-(a-b)

There are as many books on each floor: A-B = 2B-C 12B-C = 2C-(A-B).

From ①: a+c=3b②

Because a+b+c=384③.

Substitute ② into ③ to get 4b=384.

b=96

In other words, there are 96 books in the middle layer.

I wonder if I have studied equations in the fifth grade of primary school. I wish I could understand them clearly.