Question: I need a clearer understanding of how to solve: There are 50 books in a small library. Half of them are written in English, and 10% in German. All others are written in Spanish. How many Spanish books are there?
Answer: 
Half of the books are in English, which means 50 * 0.5 = 25 books.
German books make up to 10%, which means there are 10/100 * 50 = 5 of them.
So as a result there are 50 - 25 - 5 = 20 Spanish books in the library.