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This is an interview question. i am not sure how to solve such problems.

Problem:

You are given $n$ white balls in the beginning.Each day you pick up a ball randomly color it red and put it back. If it is already colored, you simply put it back. This operation is performed for $d$ days. What is the probability that after $d$ days you will have greater than $k$ balls colored?

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    You can also see this as a counting problem. In how many ways will you have greater than $k$/less than $k$ balls colored? And how many different results can you get?2012-08-12

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