The probability of choosing a green marble, not replacing it, and then choosing a red marble can be calculated by multiplying the probabilities of each event.
Step 1: Determine the probability of choosing a green marble.
In the bag, there are a total of 4 marbles. Since there is only 1 green marble, the probability of selecting it is 1 out of 4, or 1/4.
Step 2: Determine the probability of choosing a red marble.
After selecting the green marble, there will be 3 marbles left in the bag. Now, there is only 1 red marble remaining. Therefore, the probability of selecting the red marble is 1 out of 3, or 1/3.
Step 3: Calculate the probability of both events occurring.
To find the probability of both events happening, we multiply the probabilities obtained in steps 1 and 2.
(1/4) * (1/3) = 1/12
Therefore, the probability of choosing a green marble, not replacing it, and then choosing a red marble is 1/12.