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Identical Objects in Distribution
## Formula
The number of non-negative integer solutions of:
$$x_1+x_2+\cdots+x_n=r$$
is:
$${}^{n+r-1}C_{r}$$
Equivalently:
$${}^{n+r-1}C_{n-1}$$
## Usage
- Used to convert identical-object distribution problems into combinations.
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