Answer: There are 10 books in all.
Step-by-step explanation:
Let x be the total number of books.
Since we have given that
Amount he paid for college textbooks = $450
Price of each book = $75
Number of books checked out for free = 4
So, Number of books which are not free would be
[tex]\dfrac{450}{75}=6[/tex]
So, the total number of books would be
x = Number of books which are not free + Number of books which are free
[tex]x=6+4=10[/tex]
Hence, there are 10 books in all.