A pie chart shows how the 1,800 books of a school library are divided among four sections. The sector for Science measures 80° and the sector for Fiction is exactly twice as large. How many more Fiction books than Science books does the library hold?

Fiction takes $2\times 80$° = 160°, so it holds $(160/360)\times 1800=800$ books, while Science holds $(80/360)\times 1800=400$ books. The difference is 800 − 400 = 400 books. Equivalently the 80° gap between the sectors is 80/360 = 2/9 of the collection, and $1800\times 2/9=400$. The value 800 is Fiction's own total rather than the difference, and 80 and 160 are ANGLES in degrees read off as though they were counts of books.