the cost,in millions of dollars, to remove x % of pollution in a lake modeled by c= 4000/200-2x

what is the cost to remove 40% of the pollutant?

4000/(200-2*40)=$33.334 million 

what is the cost to remove 80% of the pollutant?

4000/(200-2*80)=$100 million

what is the cost to remove 90% of the pollutant?

4000/(200-2*90) = $200 million

for what value is this equation undefined?

The equation is undefined when the denominator is 0: 2x=200, or  x=100