Thabo owns exactly 140 books, and each book is either paperback fiction, paperback nonfiction, or hardcover nonfiction. If he owns 20 more paperback nonfiction books than hardcover nonfiction books, and twice as many paperback fiction books as paperback nonfiction books, how many hardcover nonfiction books does Thabo own?

The no. of hard cover non-fiction, x = 20 books

Solution:

Total no. of books owned by Thabo, t = 140 books

Let the no. of hard cover non-fiction books be x

Then the no. of paper back non-fiction books, y = x + 20

According to the question the no. of paper back fiction books = 2 (x + 20)

Now,

x + y + 2 (x + 20) = 140

x + x + 20 + 2x + 40) = 140

4x = 80

x = 20