If you are trying to fill a 20 gallon tank with a 2 gallon bucket you would need 10 buckets (2 x 10 is 20). So to clean 20 gallons of water you need 10 times what would be needed for 2 gallons. You needed 3/4 of a cup of disinfectant (10 x 3/4 is 30/4 an improper fraction). To convert 30/4 cups to a more usable mixed number you figure out the largest number less than or equal to 30 that is divisible by 4. That number is 28 leaving 2/4 (28/4 + 2/4 is 30/4). Because 28 is divisible by 4 28/4 can be turned in to a whole number 7. The rest 2/4 needs to be simplified, both 2 and 4 are divisible by 2 (2/4 is 1/2). The result is 7 1/2 cups of disinfectant(10 x 3/4 is 7 1/2).