How do you find how many cups are in a gallon

First we need to find the volume of water that can be filled in water 
   Then we will use the equation that
                  1 gallon = 3.8 lires
Then by using this we will calculate the amount of cup used by equalizing with litres and then by gallons and then calculate number of cups in gallons

Do an experiment to test out if the Capacity Table is true.
Or use mathematics, 1 gallon is 16 cups. To find it use from the smallest capacity to gallons.