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Read the following SAT test question and then click on a button to select your answer.

1、A jar contains only red marbles and green marbles. If a marble is selected at random from the jar, the probability that a red marble will be selected is . If there are 36 green marbles in the jar, how many red marbles are there in the jar?

Answer Choices

　　(A) 18

　　(B) 24

　　(C) 54

　　(D) 72

　　(E) 108

Explanation:

The correct answer is D

Choice (D) is correct. Let r be the number of red marbles in the jar. The probability of selecting a red marble is equal to the number of red marbles divided by the total number of marbles. Since the probability of selecting a red marble is , it follows that . There are 36 green marbles and r red marbles, so the previous equation becomes . This simplifies to2(r + 36) = 3r, or 2r + 72 = 3r. Therefore, the number of red marbles, r, is 72.

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