I'm not sure I've seen this puzzle before, and I really like it:

We have n keys and n boxes. Each key fits only one box. We shuffle the keys and put one in each box. Then we randomly break open 1≤k≤n boxes. What is the probability that we can unlock all the other boxes?

If I understand this correctly, then it is zero unless $k>=n/2$. Since otherwise you dont have enough keys, right?


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