Mass of empty water tank is 3.64 kg. Some amount of water is filled into the empty water tank. After filling of water upto certain level, the mass of the water tank becomes 51.8 kg. Mass of empty water tank + Mass of water = 51.8 kg. Hence, Mass of water present in the tank is equal to difference in the mass of water tank with water and mass of the empty water tank.
Hence of mass of water filled in the tank is equal to (51.8 - 3.64) kg =48.16 kg.