there are 10 symbols ‘C’ and 3 partition walls, represented by a ‘+’ sign. That is, there are n-1+k = 13, equivalently n+k-1, symbols. Further note that each of the 3 partition walls could be in 1 of 13 positions. In other words, to represent various choices of 10 candies from 4 categories, the positions of the partition walls could be rearranged by choosing n-1 = 3 of n+k-1 = 13 positions
Read full article from Alexander Kruel · Number of combinations with repetition
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