A bag contains one billiard ball. It can be white or black (with equal probability). We put a white ball inside the bag (so now there are 2 balls in the bag).
Now we take one ball from the bag. It turns out the the ball we took is white.
Question: What is the probability that the ball left in the bag is black?
My opinion: There is $1/2$ probability that there is white ball in a bag, then we add white ball to the bag so the probability increases to $3/4$. Now if we take a ball from it and it turns to be white it means... I do not know what it means.