MARBLES problem wrong answer and runtime error

#1

HI!!
I think the answer is just (n-1)C(n-k). But i get wrong answer and even runtime, Do i need to modulo my answer at any stage?

#2

You are wrong with your mathematical formula here. Think again, and share your thinking.
Then I’ll tell you the right way :slight_smile:

#3

I think ki the answer is same as number of ways of solving this problem:
x1+x2+x3+x4…+xk = n-k;
I did -k with n because atleast one of each need to be there. Why is this wrong?

#4

See, if we choose n balls, then k balls have to be of k different colours for sure.

We now have to select n-k balls with k types of options.
There is a general formula for this case (Using PnC) :
(n+r−1)! / (n−1)!⋅r!

Put the new value (n-k) instead of n in the mathematical formula to get the New formula for our question that equals to C(n-1,k-1).
I hope i am clear and you shall be able to implement the code :slight_smile:

#5

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