3% is pollutant.
After adding fresh water, let the vol. of the tank be Vf and pollutant drops to 0.5%. Since the amount of pollutant remains same before and after adding water, we will take a material balance of the pollutant.
(3/100)*Vi = (0.5/100)*Vf
We need to know amount of clean water added, i.e. Vf-Vi
Now, initially we had 2000 gallons of water. after addition of pollutant it becomes 97%.
therefore, 97% of Vi = 2000 gallons.
First solve for vVi, then Vf and finally Vf - Vi to get the answer.