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 travelsxfeet.

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?