You have read 28 books from the ' crazy silly school ' series. If there are a total of 45 different books in the ' crazy silly school ' series How many more do you still have to read?
There are a total of 45 different books in the 'crazy silly school' series. 
You have read 28 books from the 'crazy silly school' series. 

That means that you have read 28 out of 45 books. 

To find out how many more you have to read, we need to subtract 28 from 45. 

45-28=17

You have 17 more books to read.