Its a math problem. 0.2 'parts' chlorine per 'million parts' water.
To make it easier, multiply both by 5, and you have 1 part chlorine to 5,000,000 parts water.
That's 1 gallon of chlorine per 5,000,000 gallons of water, or 1 cup chlorine per 5,000,000 cups of water. Whatever the measurement, the numbers stay the same.
A dripping faucet wastes water, and they say one gallon has has 90,840 'drops' of water.
So, 1 drop of chlorine in 5,000,000 drops of water. Just figure out how much water is 5,000,000 drops by dividing it by 90,840. It turns out to be one drop per approximately 55 gallons of water.