When kerosene is purified to make jet fuel, pollutats are removed by passing the kerosene through a special clay fliter. Suppose a filter is fitted in a pipe so that 15% of the impurities are removed for every foot that the kerosene travels.
a. Write an exponential function to model the percent of impurtiy left after the kerosene travels x feet.
My issue is coming to understand why the function is f(x)=0.85^x. What's the reasoning behind subtracting .15 from the whole before beginning to travel?