Please send answer in detail

Let the length of the side of the cube be 'l' meters 

Volume of the cube = L³ 
The area of each face of the cube is L² 
Considering all the 6 faces of the cube, the weight of the cube is  
= 6 × L² × 8000 kg/m³ × 0.001 m  
=  L² × 48 kg/m² 

Weight of the water displaced by the volume of the cube 
=  L³ × 1000 kg/m³ 

Condtion for the block not to sink in water is
Weight of the cube = Weight of the water displaced by the volume of the cube 
L² × 48 kg/m² = L³ × 1000 kg/m³ 
On solving we get
L =  48 kg/m² / 1000 kg/m³ 
L = 0.048 m 
L = 48 mm