I want to prove the combinatorical identity:
They both are ‘$r$ identical balls to $n$ different cells with repeats and without order’.
But for the left side I see it as ‘choosing $k$ not empty cells and put in them one ball in each, and then all the other $n-k$ balls put into the choosen $k$ cells with repeats and without order’.
But for the second part isn't it
I don't see how it is: