When the cube is placed in the water, the buoyant force is the volume of the cube displaced by water.
The volume of the cube is V = [48 cm ( m / 100cm)]3 = 0.1106 m3
The buoyant force = (Density of water * Volume of cube) ( g )
= [1000 kg/m3 ( 0.1106 m3)] 9.81 m/s2 = 1084.98 kg-m/s2 = 1085 N
For the cube to float, the buoyant force has to equal the weight of the cube
Weight of cube = buoyant force = 1085 N