“If you have an old version you’re good on all of the content, except the questions at the end will be different. So you’ll need a new book to do the homework.”
Or go to the library and photocopy the questions. I had a math instructor literally tell us to buy the old edition and photocopy the questions from the library.
The usual trick is your math teacher was commissioned to write/update one chapter of said book, incentivizing him to push new versions every semester.