Hi, I am not able to comprehend the solution of a problem from a puzzle book. Your friend shows you an urn and tells you that there is one marble in the urn. The marble is either black or white. (Assume that the marbles are identical in shape and any two marbles with the same color are indistinguishable.) She then drops one black marble in the urn. Then sticks her hand in the urn and takes out one marble randomly, which turns out to be black. Now she asks you what are the chances of drawing a black marble from the urn. According to the book the probability of drawing a black marble is 66.6%. My answer was 50%. Here is my logic. 1. Assume that initially there was a black marble (b1) in the urn. So when the friend drops another black marble (b2), the urn has b1 + b2. Now when she takes out one black marble , the probability of drawing another black marble is 100%. 2. Now assume that initially there was a white marble (w1) in the urn. So when the friend drops another black marble (b2), the urn has w1 + b2. As she takes out a black marble, it has to be b2. So the probability of drawing a black marble is 0%. Therefore, my answer is the average of the 100% and 0% chances.