I currently work as a tutor for high school kids - so you can assume that this is commerical. To give them homework, I take questions from textbooks, past exams, and from online. I reword the questions and then redraw all the diagrams and then give it to them.
I've only just realised that this is exposing me to potentially a lot of liability due to copyright violation. But I can't possibly make this many questions myself that are of a high quality.
My question is, how much do I need to modify the question until it is no longer copyrighted? How much rewording is enough - if ever? Is redrawing the same diagram myself averting copyright? I've read things online that copyright cannot protect ideas, which in this case I would presume to be the "tricks" or "essence" of the question - and so the only copyrighted thing are the words and diagrams?