Let's assume that the total number of marbles in the bag is "x".
Given that the probability of choosing a black marble is 3/5, we can conclude that there are 3/5 of "x" black marbles in the bag.
Since the probability of choosing a white marble is equal to the probability of choosing a green marble, we can conclude that each of them is equal to 1/5 of "x".
So, the number of black marbles is 3/5 of "x", which is (3/5)x = 9.
To find the value of "x", we can solve the equation:
(3/5)x = 9
To isolate "x", we can multiply both sides of the equation by the reciprocal of 3/5, which is 5/3:
(3/5)x * (5/3) = 9 * (5/3)
x = (9 * 5) / 3
x = 15
Therefore, there are 15 marbles in the bag.
Answer: \boxed{15}.