Hello, KK!

I hope your sister is familiar with factorials . . .

Yes, there is a formula . . .There are 6 books that I want to buy, and I can only get 4 of them.

How many different combination of books are there for me to choose from?

This is a Combinations problem -- the order of the books is not important.

Choosing 4 items from 6, there are: differemt choices.