So if Lisa's reading list is 20% fiction, that's 20/100. So how can you simplify 20/100 so that the top number (the numerator) is 5?
If you divide both numbers by 4 (the same as dividing the fraction by 4), then you get 5/25. That means 5 out of 25 books are fiction. Her reading list is 25 books long.
