Answer: He has already read 42 pages
Explanation:
First, we need to find the total number of pages in the book
To do that, from the question we were told Elvin read 3/7 of the pages, if he read 3/7 of the pages, this implies that he has 4/7 pages left to read.
Let x = the total number of pages in the book
So the mathematical interpretation of this statement is;
4/7 of x = 56
4/7 × x = 56
= 56
cross multiply
4x = 56 × 7
4x = 392
Divide both-side of the equation by 4

x = 98
The total number of pages in the book is 98 pages
To get the number of pages read, we simply subtract the number of pages left to read from the total number of pages.
Number of pages read = Total number of pages - Number of pages unread
Number of pages read = 98 - 56 = 42
Therefore the number of pages he had already read is 42 pages