density is defined as the mass per unit volume,
mass of the solution is therefore
mass = density x volume
1 gal = 3785.41 mL
therefore 14.1 gal = 3785.41 x 14.1 = 53 374.3 mL
mass of gasoline required = 0.737 g/mL x 53 374.3 mL
mass of gasoline required to fill the tank - 39 337 g = 39.3 kg
39.3 kg of gasoline is required
