Agraduated cylinder contains 20.0 ml of water. an irregularly shaped object is placed in the cylinder, and the water level rises to the 31.2-ml mark. if the object has a mass of 7.42 g, what is its density?

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

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water