This math question can be solved by figuring out the relation between the increase in the volume of gallons of water in the tank and the increase in volume of disinfectant. To figure this out, one must understand how much of an increase there is in rate between 4 gallons of water and 20 gallons of water. 20 gallons of water is five times the amount of 4 gallons of water. Therefore, 4 cups of water multiplied by 5 cups of water is equal to 20 cups of water.
As such, if 3/4 of a cup of disinfectant is required to clean 4 gallons of water in a tank, then 5 times the amount required for the 4 gallons of water will be needed to clean 20 gallons of water in the tank. Therefore, one would need to multiply 3/4 by 5, since there needs to be an equal rate of increase between the amount of water and the amount of disinfectant. 3/4 times 5 equals 3.75. This means that in order to clean the 20 gallons of water in the tank, one must use 3.75 cups of disinfectant.