ANSWER:
a) 50%
b) 80%
c) It is not possible to remove all the contaminants
Explanation:
We have the following function:

Where C would be the cost and x the percentage of removal, therefore:
a)

b)

c)
It is not possible to remove all the contaminants, due to the function nothing, since the moment that x is equal to 100, the denominator is 0, which causes a mathematical indeterminacy