You have read 22 books from the ' crazy silly school ' series. If there are a total of 42 different books in the ' crazy silly school ' series How many more do you still have to read?
There are 42 books in the series. 
You have read 22 books. 
That means you have read half of the series. 
That means you still have to read half of the series, or 21 more books.