ShmoopTube

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Here’s your shmoop du jour, brought to you by Marbles.

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If found, please return to… oh, we can’t even remember where.

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A bag contains yellow, blue, and red marbles.

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The probability of drawing a red marble is 1/4 and the probability of drawing a yellow marble is 3/5.

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If there are 100 marbles in the bag, how many are blue?

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And here are the potential answers…

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To solve this problem, we just have to figure out what fraction of marbles are blue.

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Okay, we’re given the probability of drawing a red and yellow marble, so we can figure

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out how many red and yellow marbles are in the bag.

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We start with 100 marbles. 1/4 of them are red.

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To find how many marbles that is, we just have to find 1/4 of 100.

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That’s easy, multiply the two, and voila, that’s it.

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25 marbles are red.

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Now for yellow. The problem tells us that 3/5 of them are yellow.

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Again, we multiply 3/5 times 100 and we get 60.

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Since there are 25 red marbles and 60 yellow marbles and all the rest are blue....

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...we get 25 plus 60 is 85.

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Then, just subtract that 85 from 100, and we get 15 are blue.

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So our answer is (B).

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