The total number of marbles in the bag is 5 (pink) + 7 (red) + 12 (green) + 2 (blue) + 4 (black) = 30 marbles.
The theoretical probability of picking a green marble can be calculated as:
Number of favorable outcomes (green marbles) / Total number of possible outcomes = 12 / 30 = 2 / 5
Therefore, the theoretical probability of picking a green marble is 2/5 or 0.4.