A 30ml solution with a mass of 45g has a density of 45/30 or 1.5 g/ml, while a 40ml solution with a mass of 50g has a density of 50/40, or 1.25 g/ml. The 30ml solution with a mass of 45g has a higher density.

Assuming that the water in the can is pure water (ie. with a specific gravity of 1) then the mass of the water in the can is 5.5kg (1L of pure water has a mass of 1kg), leaving the mass of the can to be 850g.