Which one is more expensive?

It's the first day of my problem-solving  course with Mr. Ucup. I actually like his style of teaching and the course since I never got this course when i was at undergraduate.

Here is an example of problem solving in mathematics I got today;
Budi bought 3 pens and 2 books worth Rp. 30,000
Budi also bought 2 pens and 2 books worth Rp. 25,000

Then the question is which one is more expensive? pen or book?

Here is a simple answer without calculating each price,

We can see that both of them have 2 pens and 2 books, if we just add/buy one more pen it costs Rp. 30,000 and if we add/buy one more book, it costs Rp. 25.000.

Conclusion:
 It's clear that a pen is more expensive Rp. 5,000 than a book.