A means arrangement. Arranging is like queuing. Objects are ordered.

C means combination, just like fried rice with eggs and fried rice with eggs. These objects are disordered.

Because of its different meanings, the calculation method is close to:

A(x，y)=y！ /(y-x)！

C(x，y)=y！ /(y-x)！ *x！

Where y> = X.

A deep understanding of concepts is a good way to solve scientific problems logically. What is depth? It depends on your own understanding.