You have read 3 books from the ' crazy silly school ' series. If there are a total of 39 different books in the ' crazy silly school ' series How many more do you still have to read?
There are a total of 39 different books in the 'crazy silly school' series. 
You have read 3 books from the 'crazy silly school' series. 
That means you have read 3/39, or, 1/13 of the total books in the series. 
That also means that you have 12/13, or, 4/3 of the books left to read. 
That's a total of 36 books left to read.