The probability of randomly picking out a red marble is 50%, or 1/2.
You can figure this out by finding out the percentage of each marble in the bag, and that percentage would be the probability for picking out that colour of marble. For example, the percentage of red marbles in the bag is 50%, the percentage of blue marbles in the bag is 20%, and the percentage of green marbles in the bag is 30%.
By doing this, you figured out not only the percentage of each colour of marble in the bag, but you also figured out the probability of each marble being randomly picked out of the bag.
