"How many combinations of the same objects were purchased?"
Sorry for complaining at every challenge and propably it is a just an issue of my bad English, but I can't see how the solution gives an answer to the question on top.
The solution, which I looked at after desperate thinking, gives an answer to the question: How many purchases are unique or in other words don't have another purchase with the same products?
At least the "the" in "How many combinations of the same objects were purchased?" should be deleted because in the solution there are many different combinations of objects that have a common combination, not only one. I hope it is not only because of my bad English that I misunderstood the question.