Answer:
The number of pages he already read is:
  70 pages
Explanation:
Let x be the total number of pages in the book.
It is given that:
Leon read 5/7 of the pages in his book. 
This means that the number of pages that are left to read in the book are:
 
 
Also, it is given that:
He has 28 pages left to read.
i.e.

Hence, the number of pages he already read is:

  Hence, the answer is:
 70