There are 960 pages because 80 x 12 = 960. Each book is 160 pages because 960 / 6 = 160.
This justifies what answer for what question? Q & A:
Jack has a stack of books that is 12 inches thick. He knows from experience that 80 pages is one inch thick. If he has 6 books, how many pages is each one on average?
160