If four marbles are picked at random, what is the probability that one is green, two are blue and one is red?

Ways of selection of 4 marbles = n(S) = ¹⁵C_{4
Ways of selection of one green marbles = n(E1) = ²C1
Ways of selection of two blue marbles = n(E2)=⁴C2
Ways of selection of one red marble = n(E3) = ⁶C1
∴ Required probability = (²C1 x ⁴C2 x ⁶C1) / (¹⁵C4) = 24/455}