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Here is the problem: If 4 pens cost 6, how much do 10 pens cost?

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Proportion: 4 pens cost 6.00; find the cost of 10.

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Read the problem. 4 pens for 6.00.

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Unit price. 6.00 divided by 4 equals 1.50 per pen.

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Scale to the new quantity. 10 times 1.50 equals 15.00.

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So the answer is 10 pens cost 15.00.

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Find the price of one unit, then multiply by the quantity you need.

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15.00 divided by 10 equals 1.50 per pen, the same unit price, which checks out.