I wonder if you can use 15% from each "edition" (since they're all basically the same anyways).

No. The law generally doesn't stand for "aha, but what if I ..." tricks.

It's an interesting question though. Because the publishers will invariably argue that each edition is different. If the editions are different, then surely you can get 15% of each.

They only argue that they are different for face-saving reasons ("we're not milking you, honest!"). There's no legal reason they need to be different.

How would you be able to pick and cover specific covers without buying the book?

The library, potentially. The other option that I've seen is that the professor buys one copy of the books and then copies the parts they want so they can distribute to the students.

Oh, okay.

The teacher/professor would be able to buy the book and can then copy and hand out the parts they are actually using for their classes.

A smart professor is the one who wrote the book they sell to their own class.

Right. Not only does that bastard Gil Strang keep updating with new editions, he also is the publisher!