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Nonlinear functions SAT question

Math · Advanced Math · Medium · Answer it here, explanation included.

From 2005 through 2014, the number of music CDs sold in the United States declined each year by approximately 15% of the number sold the preceding year. In 2005, approximately 600 million CDs were sold in the United States. Of the following, which best models C, the number of millions of CDs sold in the United States, t years after 2005?

ExplanationShow

Choice B is correct. A model for a quantity C that decreases by a certain percentage per time period t is an exponential equation in the form C equals, I times, open parenthesis, 1 minus, the fraction r over 100, close parenthesis, to the t power, where I is the initial value at time t equals 0 for r% annual decline. It’s given that C is the number of millions of CDs sold in the United States and that t is the number of years after 2005. It’s also given that 600 million CDs were sold at time t equals 0, so I equals 600. This number declines by 15% per year, so r equals 15. Substituting these values into the equation produces C equals, 600 times, open parenthesis, 1 minus the fraction 15 over 100, close parenthesis, to the t power  , or C equals, 600 times, open parenthesis, 0 point 8 5, close parenthesis, to the t power.

Choice A is incorrect and may result from errors made when representing the percent decline. Choices C and D are incorrect. These equations model exponential increases in CD sales, not exponential decreases.

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