Chapter 4 - Exponential Functions - 4.1 Introduction to the Family of Exponential Functions - Exercises and Problems for Section 4.1 - Exercises and Problems - Page 146: 35

$P(n)= 420\cdot (0.15)^n$

The problem describes an exponential function with the following parameters. \begin{equation} \begin{aligned} a&= 420\\ r&= 85\%= 0.85\\ b&=1-r = 0.15 \end{aligned} \end{equation} Hence, a model of the remaining impurities in the water is given by: \begin{equation} \begin{aligned} P(n)&= a\cdot b^n\\ &= 420\cdot (0.15)^n\\ \end{aligned} \end{equation}