Suppose that the cost (in millions of dollars) to remove $x$ percent of a certain pollutant is given by the cost-benefit function
$$f(x)=\frac{20 x}{102-x} \text { for } 0 \leq x \leq 100.$$
(a) Find the cost to remove $85 \%$ of the pollutant.
(b) Find the cost to remove the final $5 \%$ of the pollutant.