Saturday 20 February 2016

paradoxes - Balls and Urn Paradox

So, I came across the following paradox:




At $1$ minute before noon, put in balls $1 \sim 10$ and take out ball number $1$. At $1/2$ minute before noon, put in balls $11\sim20$ and take out ball number $2$ and so on. How many balls are there at noon?



None.



At $1$ minute before noon, put in balls $1 \sim 10$ and randomly take out a ball. At $1/2$ minute before noon, put in balls $11\sim20$ and randomly take out another ball and so on. How many balls are there at noon?



None.



Okay, so I understand the first paradox because one can describe the exact moment each ball was taken out. But, you can't apply a similar argument to the second paradox because we randomly take out a ball.
I feel as if it's like infinitely summing $\frac{1}{n}$ and eventually there would be too many balls.




Can someone explain to me mathematically why this is the case?

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