There are $n$ balls in the box. There are only black or white balls in the box. The number of white balls is unknown. Assume that that any number of white balls is equiprobable and find a probability to take a white ball from the box at the second time if the first ball that was taken is white. Balls are taken from the box randomly.

Question

anonymous · Answer

again?

anonymous · Answer

oh maybe there is a different question this time?  the set up is the same, but there is no question here

klimenkov · Answer

"... find a probability to take a white ball from the box at the second time..."

anonymous · Answer

aaaah i thought maybe something different. 
no replacement i assume, and we don't know the first ball right?

anonymous · Answer

what do you want to bet it is one half?

anonymous · Answer

I really don't see how anyone is supposed to come up with a sensible answer to this....

klimenkov · Answer

"...the first ball that was taken is white...find a probability to take a white ball from the box at the second time..."

anonymous · Answer

I repeat  "I really don't see how...."  :-)

klimenkov · Answer

What about http://en.wikipedia.org/wiki/Bayes%27_theorem

anonymous · Answer

How does that help?

klimenkov · Answer

Do you know where to use this theorem in general?

anonymous · Answer

I might use Bayesian inference if I had some idea of the percentage of white balls to start with....

klimenkov · Answer

Do you know the formula of total probability?

anonymous · Answer

U mean this http://en.wikipedia.org/wiki/Law_of_total_probability

anonymous · Answer

The problem here is that there is insufficient data on which to base any answer to the question.

anonymous · Answer

Any number of white balls is equiprobable, but what about the number of black balls?
Is the number of black balls known?
Can't find a numerical answer here, but I suspect an algebraic expression at least is possible.

anonymous · Answer

|dw:1351374001717:dw|