Answer:
I think 87.2% is the correct answer.
I think it will help you

Thickness of the book is ~
Refer to the attachment for solution ~
Given:
The equation is:

To find:
The value of a.
Solution:
We have,

On simplification, we get




On comparing both sides, we get




And,




Therefore, the value of a is 2.
Four? Isn't it stated in the question? Unless I am assuming wrong.