There are 3 + 3 + 6 = 12 marbles in the bag and 12 * 12 = 144 ways to choose two marbles when replacing. There are 3 * 6 = 18 ways to choose a red marble and then a blue marble so the probability is 18 / 144 = 1 / 8.
The correct answer is D. Choose two points on the graph and count the rise and the run between them. Graph D is the only one that also has a slope of 3/4
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