ICSE Class 9 Answered

Side of the cube, a = 5 cm

Volume of metal cube, V = a³ = (5 cm)×3 = 125 cm³ = 125 x 10⁻⁶ m³

Density of metal, d = 9 g/cm³ = 9000 kg/m³.
Mass of the metal cube m = volume x density 
                                    = 125 x 10⁻⁶ m³×9000 kg/m³
                                    = 1.125 kg
Weight of the metal cube in air, W = mg = 1.125 kg x 10 m/s² = 11.25 N
The cube immersed in a liquid of density = 1.2 g/cm³ = 1200 kg/m³
Volume of liquid displaced = Volume of metal cube = 125 x 10⁻⁶ m³

Mass of liquid displaced = Volume x density of liquid 

                                 = 125 x 10⁻⁶ m³×1200 kg/m³
                                 =  0.15 kg
Weight of liquid displaced = 0.15  kg x 10 m/s² = 1.5 N = upthrust

Weight of metal cube in liquid = Weight of the metal cube in air - upthrust
                                                = 11.25 − 1.5 
                                                =  9.75 N
Therefore tension in the string = 9.75 N