What's the probability?

Question

in a box, there are 2 balls (we dont know their colors). a white ball is added to the box. a ball is picked at random from the box: what is the probability that the ball picked is white???

em... please explain your answer!

Answer 1

Other than saying "at least one third", we can't say. You have not limited the colors of the other balls in any way.  If you had said that they could be either black or white, we could have made a start at estimating.

Answer 2

Oikos is correct.  The probability is at LEAST .333%.  The probabilitty will be MORE if there happen to be more white balls in the box.  But as you mentioned the colors of the other balls are unknown.

Even if there are no other white balls, the probability can never be LESS than 1/3.