Xiaofeng is known to have 36 books, of which 6 are lent to Xiaomei and 9 to Xiaogang. Number of books borrowed by Xiaofeng = number of books lent to Xiaomei+number of books lent to Xiaogang =6+9 = 15. A: Xiaofeng borrowed 15 books.

Mathematics is a universal means for human beings to strictly describe the abstract structure and mode of things, and can be applied to any problem in the real world. All mathematical objects are artificially defined in essence. In this sense, mathematics belongs to formal science, not natural science. Different mathematicians and philosophers have a series of views on the exact scope and definition of mathematics.