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Problem-Solving and Data Analysis / Percentages Difficulty: Medium

The number of coins in a collection increased from $9$ to $90$ . What was the percent increase in the number of coins in this collection?

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Explanation

Choice D is correct. It's given that the number of coins in the collection increased from $9$ to $90$ . It follows that the number of coins in the collection increased by $90 minus 9$ , or $81$ . Let $x %$ represent the percentage that $81$ is of $9$ . The value of $x$ can be found using the proportion $\frac{81}{9} = \frac{x}{100}$ , or $9 = \frac{x}{100}$ . Multiplying both sides of this equation by $100$ yields $900 equals x$ . Thus, when the number of coins in the collection increased from $9$ to $90$ , the percent increase was $900 percent sign$ .

Choice A is incorrect and may result from conceptual or calculation errors.

Choice B is incorrect and may result from conceptual or calculation errors.

Choice C is incorrect and may result from conceptual or calculation errors.