If you look at old books like Euclid, and Archimedes it seems that there weren't any problem set exercises. They just presented the all information. There were no "end chapter" problems. In short, it seems that most ancient books were written as treatises.
However, for contemporary textbooks, they often leave exercises at the end of the chapter, often with no solutions. I wonder when did exercises were appended into textbooks. Specifically, exercises which do not contain answers/derivations.
I conjecture maybe from the the industrial revolution, but that's just my guess. I want to know the palpable answer to this. In other words, I wonder when did books begin to include exercises with omitted solutions.