According to Archimedes principle, when an object is submerged in water, it displaces a volume of water equivalent to the volume of the object. Therefore, the object's volume is:
31.2 - 20 = 11.2 mL
Next, we know that the density of any substance is the ratio of the substance's mass to its volume, so the density is:
Density = mass / volume
Density = 7.42 / 11.2
Density = 0.66 grams per mL
