Three people, if every two people give each other a card, a * * * gives each other three cards.

So a * * * sends six greeting cards, which is a very simple math problem.

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.