The total number of marbles in the bag can be found by adding the quantity of each type of marble together. So we have 2 red marbles, 3 green marbles, and 2 black marbles, which totals to 2 + 3 + 2 = 7 marbles.
The probability of an event happening is calculated as the number of outcomes in which the event happens out of the total number of outcomes. In this case, we want the probability of drawing a black marble, and we know that there are 2 black marbles out of a total of 7 marbles.
So, the probability of drawing a black marble is the number of black marbles divided by the total number of marbles, which is 2 / 7. Thus, the probability of drawing a black marble from the bag is approximately 0.2857142857142857, or 2 out of 7 if written as a reduced fraction.