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Solve the problem, then explain how you got your answer.
Madeline has red, blue, and green marbles. There are 28 marbles in all. 8 of them are blue and there are 3 times as many red marbles as green. How many red marbles are there?
She would have 15 red marbles, because 28 - 8 = 20, and there are 3 times as many read as green. 5 + 15 + 8 = 28.