When we draw with replacement we have independent events and the probability of the conjunction is the product of the individual probabilities.
P(Green first & Blue second) = P(Green first) P(Blue second) = P(Green) P(Blue)
P(Blue) = P(Green first & Blue second) / P(Green) = (2/25)/(1/5) = 2/5
So 2/5 of the marbles are blue, (2/5)(15)=6 marbles
Choice D
