Since there are four marbles in the bag to begin with, that starts us off with 1/4 as the probability, the one being the green marble, while the four is the total number of marbles.
Since the green one wasn’t replaced, the probability stays at 1/4, the one being the red and the four being the total number of marbles.
1/4 x 1/4 = 1/16
Hope this helps, and I apologize if I misunderstood!
