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So I fully understand:

Permutations without repetitions: $\dfrac {n!}{(n-r)!}$

Permutations with repetitions: $n^r$

Combinations without repetitions: $\dfrac{n!}{(n-r)!r!}$

But I'm having problems finding resources online that visually explain combinations with repetitions: $$\dfrac{(n+r-1)!}{(n-1)!r!}$$

I would be very grateful if anyone could give me a good visual, step by step explanation of how they work. Maybe use the case of choosing $3$ letters from a set of A, B and C, with repetitions allowed.

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