If a foam cube 20 cm on a side has a density of 300kg/m^3 is placed in fresh water, how much of the cube will be underwater?

6 cm

Explanation:

The weight of the water displaced equals the weight of the cube.

mg = ρVg

m = ρV

The mass of the cube is its density times its volume, and volume of a cube is the side length cubed.

The volume of the water displaced is the area times the depth.

(300 kg/m³ * (0.20 m)³) = (1000 kg/m³) ((0.20 m)² h)

2.4 kg = 40 kg/m h

h = 0.06 m

h = 6 cm

So 6 cm of the cube will be underwater.