So first add all of the marbles together: 3+2+6+9=20 Because there are 20 marbles together, that would be your denominator. All of the other numbers would be your numerator. So you would have:
3/20 green marbles 2/20 yellow marbles 6/20 blue marbles 9/20 red marbles
So your answer would be the probability of picking a red marble from the bag is 9/20 or 45%
