One of the possible solution is taking $X_1,X_2,X_3,\ldots,X_r$ as boxes and no. of objects in these boxes as $0\leq X_1\leq X_2\leq X_3\leq \cdots\leq X_r$ where $[X_1+X_2+X_3+\cdots+X_r=N]$. But how to find this? Some more Methods may also be there.
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