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I find it difficult to understand the logic and the appropriate usage of the formula:

$\dbinom{N+K-1}{N}$

I don't really understand what's posed behind the scenes of that one.

So I have some example for an exercise which requires the usage of this formula, but I know only to substitute robotically numbers without any understanding.

* How many ways we can insert 16 indistinguishable balls to 4 drawers such that in every drawer we have at least 3 balls?

So I know we should insert at the beginning 3 balls to every drawer and the remainder is 4 balls.

How do I continue and use this formula? I want to understand the formula.

Thanks.