At a garage sale, Jeff sold 80% of his books, which include both hardbacks and paperbacks. He sold an equal number of each of the two types of books, selling all paperbacks for $1 each and all hardbacks for $3 each. How many books did Jeff own before the sale?

[1] Jeff's total revenue from the sale of his books was $32.

[2] Exactly two hardbacks and two paperbacks remained unsold after the sale.



[spoiler]Source: https://www.west.net/~stewart/gmat[/spoiler]