Task: Question Answering
Input (English): Problem: thabo owns exactly 180 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 books nonfiction books does thabo own ? 
Options: a ) 10 , b ) 20 , c ) 30 , d ) 40 , e ) 50
Output (English): c