You have read 20 books from the ' crazy silly school ' series. If there are a total of 30 different books in the ' crazy silly school ' series How many more do you still have to read?
There are a total of 30 different books in the 'crazy silly school' series. 
You have read 20 books from the 'crazy silly school' series. 

This means that you have read 2/3 or 20/30 of the total number of books in the series. 

This also means that you have 10/30 or 1/3 of the total number of books in the series left to read.