A bag holds black, white and green marbles. If one marble is randomly chosen from the bag, the probability that it is black is 3/5. The probability that it is white is equal to the probability that it is green. If there are 9 black marbles, how many marbles are in the bag?

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}.