A bag has 9 red marbles and 6 green marbles. How do you find the probability of drawing a green marble, replacing it, and drawing another green marble?
First, we have the probability of drawing a green marble as 6 out of (6+9), which is
After that, we replace the green marble into the bag. At this moment, the bag becomes the same as it is before a green marble is drawn, i.e. there are still 6 green marbles and 9 red ones.
If we are too draw another green marble, the same probability,
As both requirements have to be fulfilled, the required probability is the multiplication of both,