Chapter 7 - Exponential and Logarithmic Functions - 7-1 Exploring Exponential Models - Practice and Problem-Solving Exercises - Page 441: 46

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$y=a(b^{x})$ $a$ is the initial amount. In this case $a=25000$. $b$ is the growth factor In this problem, we have exponential decay, so decreases by $15\%$ is translated to $85\%(100-15=85)$ or $0.85$ Thus, the equation is: $y=25000(0.85)^x$