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Here is the problem: A recipe needs 3 cups of flour for 12 cookies. How many cups for 30 cookies?

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Proportion: 3 cups for 12 cookies; find the amount for 30 cookies.

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Set up the proportion. 3 over 12 equals x / 30.

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Cross-multiply. x equals 3 times 30 over 12 equals 7.5.

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So the answer is 7.5 cups.

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Proportion: 3 cups for 12 cookies; find the amount for 30 cookies.

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7.5 over 30 equals 0.25 equals 3 over 12, which checks out.