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Mathematics > Standard Multiple Choice

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  green marbles in the jar, how many red marbles are there in the jar?

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### Explanation

Choice (D) is correct. Let 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  green marbles and red marbles, so the previous equation becomes . This simplifies to , or . Therefore, the number of red marbles, , is .

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