Answer: 25 books.
Explanation:
Let as consider we need to find "how many books did Mr. Wharton buy in all?"
Let total number of books bought by Mr. Wharton be x.
80% of all the books she bought were paperbacks.

It is given that the number of paperbacks is 20.

Divide both sides by 0.8.


Therefore, Ms Wharton bought 25 books in all.