2) In US 1 gallon=3785.41 mL.
In UK 1 gallon=4546.09 mL.
mass/volume=density=0.66g/mL=0.00066kg/mL. This density is also 0.66kg/L because there are 1000g in 1kg and the same number of mL in 1L.
mass=0.00066×3785.41kg=2.50kg approx in US.
mass=0.00066×4546.09kg=3.00kg approx in UK.
3) The upper layer is the liquid with density<1g/mL, that is, a and b. For c and d the upper layer is water because their densities are higher than 1g/mL.