The theoretical probability of picking a green marble can be calculated by taking the number of favorable outcomes (number of green marbles) divided by the total number of possible outcomes (total number of marbles).
Number of green marbles = 12
Total number of marbles = 5 (pink) + 7 (red) + 12 (green) + 2 (blue) + 4 (black) = 30
Theoretical probability of picking a green marble = Number of green marbles / Total number of marbles
Theoretical probability of picking a green marble = 12 / 30
Theoretical probability of picking a green marble = 0.4 or 40%