A basket contains 3redballs, 5blueballs and 7greenballs. 2 balls are picked one after the other without replacement. Find the probability that both balls are of different colors.

could be RB, RG, BG

C(3,1)C(5,1) + C(3,1)C(7,1) + C(5,1)C(7,1)
= 15 + 21 + 35
prob(your event) = 15/C(15,2) = 15/105 = 1/7