Answer:
The ratio of ozone used to the total amount of water treated per day can be calculated as follows:
Ozone used = 25,000 mg
Amount of water treated = 16,000,000 liters = 16,000,000 x 1000 ml = 16,000,000 x 1000 x 1000 mg = 16,000,000,000,000 mg
Ratio of ozone used to water treated = 25,000 / 16,000,000,000,000 = 1.5625 x 10^-11
So, the ratio of ozone used to the total amount of water treated per day is approximately 1.5625 x 10^-11.