This is a combination, choose three, that is, exclude two and leave three.

There are five choices for the first time, four choices for the second time, and 5x4=20 for * * *.

But it is possible to choose a book for the first time and b book for the second time; It is also possible to choose book B for the first time and book A for the second time.

So to eliminate duplicate selection, it is necessary to divide by 2,20/2 =10.