Change the value ot n and r. Press nCr, nPr, nHr or r!.
What is nHr? There is a situation, You have 5 same balls and 3 different bags and decide to put the 5 balls into 3 bags(You also can leave one empty bag). You have 3H5=21 ways to do it. Can you think of the formula of nHr ?
