509 gallons as 815 gallons divided by 32 = 25.468 and we multiply by 20 to get 509.375 where we round down at 509 to find full gallon, anything less (via decimal) is not a full gallon.
Lets say that you had a triangle that had an area of 64, each side being 4 (4X4X4=64) then you cut each side in half 2X2X2 the percentage decrease would be 87%